A boycott implies customers taking their business elsewhere. If this was simply people boycotting reddit there's nothing they can do.

However these are "more than customers" who are foing more than boycott. They are actively "locking the store" so others can't use it. It's not a strike (they're not withholding work) but it's also not a boycott since they are taking actual actions that lock out other customers.

Because the moderators provide (unpaid) labor for Reddit (or SO, to make the parallel with another post where you are asking the same question).

The correct term is "industrial action". Its a kind of digital strike (locking out public users) but it's not a strike in terms of labour stoppage. Hence the air quotes.