Endermen do not despawn while holding things, in my world I afk a lot around a gold farm. Basically I noticed that my performance fell way off and did a bunch of things to fix it, then I was looking around in spectator mode and saw about 100 enderman jammed into a small area. So I went to the command line and killed all enderman it killed 1043 entities. It would seem nearly every enderman that spawns picks up something (usually mushrooms). Then since 1.16 no enderman holding things despawn so it snowballs.