The reason for two full moons in one month is the Moon's synodic period, the time required for the Moon to go from one phase to the next -- say, from full moon back to full moon.
(The reason for two full moons in one month; is; the Moon's synodic period) (the Moon; to go back; from one phase)
The reason for two full moons in one month the Moon
is to go back
the Moon's synodic period
from one phase