Hi, welcome to the forum! Glad you joined.
What does your fence look like? Does it have a top rail that is a good place for the chickens to perch? What often happens is that the chickens go up there to perch and might hop down on the wrong side of the fence. So it is more or less an accident which side they hop down on. It sounds like yours have learned that there is good stuff over there so they are doing it on purpose. They may just be flying over.
This forum is filled with rules-of-thumb, old wives tales, guidelines, and suggestions. Some of them actually work but a lot of them might only work sometimes or might just make things better without totally curing the problem or may sometimes cure the problem and sometimes not. There are stories on here where clipping the flight feathers on one wing worked, there are stories where it did not. There are stories where clipping flight feathers on both wings worked, stories where it did not. When you are dealing with living animals you seldom get guarantees.
How are your chickens getting over? Are they flying to the top rail and then hopping down? Or maybe to the top of a solid post before hopping down. If you attach a piece of wire mesh so it so sticking up a foot or so that might stop them from flying over by not giving them a place to land up there.
You can try raising that fence high enough that they can't fly up there or over. If you do that I'd make the top wire mesh instead of a rail that gives them a good place to land.
The only other solution I'm aware of is to either fence them out of what you want them to stay out of or fence them in where they cannot get out.
This is a common problem and does not always have an easy solution. Good luck and once again
