This can have of course several "reasons" -and as Randy said, mental or physical.
From my expirience the most critical point is trust and self-believe-- that may sound strange for "some players".
But what I can assure to almost everyone: Let s video one guy (or more
) who are coming up on every stroke.
I video the student/player always as soon they/we want to work on this issue-- Important is, that they first see themselves, that they are REALLY jumping up/coming up during their stroke. so far so good.
What is funny: After that i ll just setup the cueball (without a object ball!) - and let them shoot just the cueball into a pocket-- And suddenly in the very most cases, they will stay down like a statue. ( and from my expirience even if they re not standing like a statue-- there will be a significant positive change/difference).
So the personal eye pattern is important- and with this example/experiment it should be also clear, that the self-beliefe is mega important, too!! As soon as they just have to shoot a cueball into the pocket, they have no fear- no doubt.
This is often an eye opener for a player/student- and they understand, that they have to LEARN to trust themselves.
Hope I was clear enough to explain the idea and sense of this example
have a nice day everyone.