Can certain things attract creepers in Minecraft?

I found this tall island with mushrooms on it in Minecraft and for the longest time no one could get to me, no creepers or anything. But built a wall around the island and all of a sudden creepers have been appearing on top of my wall even though there's so way to climb on it because it is too tall. Is there something that is attracting the creepers and how are they getting to the top of my wall when they never bothered me before?

On mooshroom biome floors mobs don't spawn but can spawn on anything else in the biome or outside of it in the water.

To stop it put light all over the walls or delete the walls