I guess I can make a distinction for the hugbox DMs that popped up (like couple of /pol/fag hugbox DMs). Those basically exist out of butthurt over people disagreeing with the group consensus and is generally an argument for removing reaction score and sticker notifs anyway.
The mass DMs though were basically giant shitposts. I think some of the earliest were born from inviting a bunch of people to some faggot's butthurt DM, and it kind of became a unique culture from there.The subsequent ones that were created to be mass DMs were usually continuing the legacy of a previous one that got deleted for some reason. Some evolved into drama hubs, some evolved into conversation hubs, but in general, they were shitposts and no one was really thinking about stickers.
I think a few people threw out the no reaction score thing as a theory as to why they're popular with some users, but to be honest, I think it's just because the guys who took part in them tended to be fun users.