The truth is most of those users were not regular mys but were and are sm stans. They've been involved in fanwars with other company stans and fandoms for years over different generations.
Every generation they'd pick a champion who they'd use to fight other fans and since back then sm groups were usually always at the very top, it wasn't risky for them to have a superiority complex over other stans.
However there was an upset during third gen when bts took over and exo were shoved to #2. These stans were livid as they couldn't have that superiority they felt from stanning a top group. As if that wasn't bad enough red velvet while being a top group were also solidly #3 and couldn't be competition for either twice of blackpink when they debuted.
This left career sm stans in a bind because they couldn't use these sm groups to fight and bully the stans of these other top groups. That's why most of them grew into bitter antis of the top3 of kpop especially bts.
Come Aespa who were smashing records in their debut year and rising ridiculously fast and suddenly a new champion was born!
The bitter sm stans got on the Aespa train and it was finally time for them to stan a group that was for sure destined for the top.
It wasn't that mys got cocky too fast but it was that old cocky fans could finally awake from their slumber.Thats why they didn't waste time coming after bts, twice and Blackpink during the height of Aespas success. These were groups they had hated for years upon years and it was finally time to use a top group against them.
Obviously that didn't last too long for them and that's why they're quiet and "humble" again.
Tldr: career sm stans success stanned aespa to continue decades long fanwars