I kept looking around, and I may have found the culprit. The twitter account stayed inactive for about 3 months since May, and the first tweet after this period received the bot likes. I would guess that coming back from inactivity triggered some bot shenanigan that made it a target to bots (like those "bots following people on instagram in the hope of being followed back").
Nonetheless, I'll keep an eye around this. If odd stuff happens again, I'll drop a message on #posh-autovote-request