Question: Why did the tower's tilt begin during construction?
About the article "Leaning Tower of Pisa", section "Abstract".

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The Leaning Tower of Pisa () or simply the Tower of Pisa ("Torre di Pisa") is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its unintended tilt to one side.
It is situated behind the Cathedral and is the third oldest structure in Pisa's Cathedral Square ("Piazza del Duomo") after the Cathedral and the Baptistry.
The tower's tilt began during construction, caused by an inadequate foundation on ground too soft on one side to properly support the structure's weight.
The tilt increased in the decades before the structure was completed, and gradually increased until the structure was stabilized (and the tilt partially corrected) by efforts in the late 20th and early 21st centuries.
The height of the tower is from the ground on the low side and on the high side.
The width of the walls at the base is .
Its weight is estimated at .
The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase.
Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees.
This means that the top of the tower is displaced horizontally from where it would be if the structure were perfectly vertical.
In January 2008, two construction cranes were moved into the construction site.
The tower's concrete core began rising in the first months of 2008.
By February 22, 2008, 9,400 of the nearly of steel necessary had been ordered.
By March 13, 2008, the steel for the tower had reached high, below street level.
From late March through early April, a tall mockup of a section of the tower's wall with twenty-four windows was tested by Construction Consulting Laboratory West in Ontario, California.
The purpose of the testing was to ensure that the all-glass exterior of the tower will be able to withstand earthquakes and extreme weather conditions.
Testing also took place on another full-scale mockup south of Albuquerque, New Mexico.
Both mockups passed the tests.
In mid-April, a batch of concrete had to be replaced after it failed a stress test.
On May 17, 2008, the tower's steel reached street level when new sections were bolted to two of the twenty-four jumbo steel columns marking the building's footprint.
The new column sections brought the height of the structure up to above street level.
In June, the chamfered steel skeleton of the tower's concrete base had begun to take shape.
By the end of the month, the concrete had been poured for the floor of the tower's basement level B3.
In his June 30, 2008 World Trade Center Rebuilding Assessment to New York Governor David Paterson, Port Authority executive director Chris Ward noted that roughly 90 percent of the construction contracts had been bid.
By August, 1 WTC had reached above street level.
During its September 16 meeting, the Port Authority board approved contracts for security and building management systems, and 95% of the contracts needed to complete the tower had been signed.
The $20 million security contract includes sophisticated video analysis in which computers would alert security personnel to abnormal situations automatically.
On October 10, Collavino Construction poured an additional of concrete for the tower's concrete core, raising it to just above street level.
By February 11, 2009, the tower was above street level.
On July 2, 2009, over of concrete were poured to form parts of the street-level plaza.
On August 13, the builders of 1 WTC set a piece of steel into place—the largest column installed yet at the building.
Each steel column, made at a factory in Luxembourg, was about long.
The columns at the bottom of the tower's foundation were about long.
By November 1, 2009, the twenty-four perimeter columns of 1 WTC were all erected, and construction of the second floor (the first floor above ground level) was nearly complete.
In addition, The Port Authority of New York and New Jersey reported in their 2009 Q3 Annual Report that steel erection should commence by January 2010, and that the typical floor construction could begin. 
The Idarkopf Tower's construction was both commenced and finished in 1980, specifically built for observation on top of the Idarkopf mountain.
Sources prove that the Idarkopf Tower replaced an older tower, used during both first and second world wars as an observation tower and as a watchtower for the German artillery. 
Construction of the tower occurred in three stages across 199 years.
Work on the ground floor of the white marble campanile began on August 14, 1173, during a period of military success and prosperity.
This ground floor is a blind arcade articulated by engaged columns with classical Corinthian capitals.
The tower began to sink after construction had progressed to the second floor in 1178.
This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning.
Construction was subsequently halted for almost a century, because the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence.
This allowed time for the underlying soil to settle.
Otherwise, the tower would almost certainly have toppled.
In 1198 clocks were temporarily installed on the third floor of the unfinished construction.
In 1272 construction resumed under Giovanni di Simone, architect of the Camposanto.
In an effort to compensate for the tilt, the engineers built upper floors with one side taller than the other.
Because of this, the tower is actually curved.
Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria.
The seventh floor was completed in 1319.
It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the bell-chamber with the Romanesque style of the tower.
There are seven bells, one for each note of the musical major scale.
The largest one was installed in 1655.
The bell-chamber was finally added in 1372.
After a phase (1990–2001) of structural strengthening, the tower is currently undergoing gradual surface restoration, in order to repair visible damage, mostly corrosion and blackening.
These are particularly pronounced due to the tower's age and its exposure to wind and rain. 
The tower's height is and has 34 floors.
Construction of the tower started in June 2005, and finished in 2007.