This decelerates it to a stop in a very short time. Obviously this means the deceleration is at a much higher rate than the acceleration due to gravity. As this is a continuous process with lots of grains of sand, the average weight is greater than the static weight. In a small time interval, subtract the weight of the number of grains starting to fall, and add the bigger force of those landing. While there is still sand in the upper half of the glass, the hourglass is topheavy and leans over against the cylinder.
Friction keeps the glass from rising.
When all the sand is in the lower half, the hourglass goes upright. No longer pressed to the side of the cylinder, it floats. Fred's explanation for the larger weight observed by the falling sand is correct. It is a problem in Halliday and Resnick's Physics. When the hourglass is top-heavy, it leans over and the friction keeps it in place.