I have a crazy head cannon, hear me out.
Ents sleep standing up. That's a thing demonstrated by Treebeard in the Two Towers. In my head, I like to think that when Ents were young and the elves were younger, young elves took part in Ent tipping. Like at night they would sneak up on an Ent and try to scare it with either fire or loud noises. I know that elves, even young ones probably wouldn't do this but it's a funny thought I had the last time I read through the books.
Also, It's something I could see Merry and Pippin doing in different times, (not war).