It means:
A.) You feet are fast causing a lot of momentum to the line, hard to stop when you're faster to the line than you're body will allow.
B.) You're dropping the shoulder, trying place the ball on the lane rather than coming through with it. This causes you to be to far forward and you losing your balance.
C.) You're not posting shots, maybe the trail leg is not touching the lane and thereby not doing its job to keep you balance. Hard to throw an object while balancing on 1 leg.
All this, with no vid to visual aid to coach is the best i can do.