Bouncing back when trying to fly *hack*

So, I've been cheating a lot on Minecraft and something has always made me curious. What causes you to bounce back when you attempt to fly? On my server I play on, I was able to fly without problem for 3 daysish. then the same issue kicked in. Where I bounce back whenever I try to use any movement hacks

Any knowledge around this?

On other servers it's definitely NoCheat, but if you have flying enabled on your server, you should be able to fly no problem.

It's NoCheat, and is near impossible to bypass (Yet few exist, e.x. BMC's fly bypass)