uhmm with the videos i cant tell much, but im pretty sure u falling off because the trailing leg is way up when it is supposed to be low and even better if u can touch the floor with it. my friends son (10yo) always have problem with that cuz he rushes to the foul line and then w/o much slide, his whole body is going forward. I always told him when hes too fast, so the best way to avoid this is going slower and stay lower (which i dont think u have any problem in that aspect)
again there are exceptions, for example WRW has something similar but works for him, try my suggestion, if doesnt work there always another solution
--------------------
When a house bowler misses the mark, misses the break point and strike, for many ppl its called a wallshot. When a pro does that its call adjustment
When a house bowler gets his finger stuck in the ball and fall on the lanes, for many ppl its called lame bowler. When a pro does that its called the Machuga flop! ha i like this one
This guy on youtube posts videos of the PBA
for ppl who dont watch TV or want to save the matches this will be great!
link! click me