I have also seen a number of students experience this. One of the more common factors is hand and foot position.
Until it becomes instinctual not to do it, some will drop a hand or foot and this causes the roll effect. Especially if they are properly weighted as far as amount.
It can also be due to weight distribution and position of the lead. I've seen people use a weight belt and not realize that a spine weight is not centered, but too far to the left or right.
I would often give students something to hold in both hand and require them to keep a grip on it with both to get out of that drop the hand habit.
Or go behind them and lift their foot.
Most don't even realize they are dropping their foot. They have to develop a sense of how that feels. Usually only takes about two pool sessions and being told to focus.
It's not something that takes a lot of time, but it does take concentration. That is something that quickie courses may not allow because so much is being jammed down their throats.