Horses also pull the reins out of a rider's hands because they want to either eat, or are thinking about doing something else too.
As said above, without video of you riding, hard to tell what is going on.
But after years of helping novice riders, I can tell you some of the most common problems is a rider with hands too high, or too tight or bouncing.
If your hands are up above waist area? Too high, ideally I like to see more in below belt area, IF you are riding trail/pleasure etc. Whole different ball of wax for Saddleseat of course. But the hands too high, cause horse's head to come up, as the reins are as the mouth is, meaning the higher the reins, the higher the head. A horse will pull against that.
Secondly, if hands aren't releasing, but are constantly engaging the bit? Horse will not be happy. That means there should be no tension in the reins, and the curb chain is not engaged.
Third. If your hands are bouncing all over the place and you are jiggling the reins? That is irritating as heck to a horse, so you need to steady the hands.
I also have seen some of this happen because of heavy hands, where you are yanking, pulling, and have death grip.
Or also because of bad hands, the curb chain is too tight, and engaging at every bumble. When I teach, I loosen the curb chain to avoid this entirely.
Practice carrying hot cups of coffee around house, as that will teach you to lighten your hands, and disengage elbows and shoulders from wrists.
Horses make me a better person.