It's phrasal verb time! Every English learner hates phrasal verbs: there is no particular rule to learn when it comes to phrasal verbs, and no way for teachers to teach them either... unfortunately, the only way to learn phrasal a phrasal verb is just to memorise it.
❓What does this phrasal verb mean? To gang up on... [someone/ something]
Example: 'All my friends agreed that I should study harder. I felt like they were ganging up on me'.