Okay, now we know what's happening:
If you boost something in Masto, the person replying, by default @'s the person boosting it.
That's actually a nice feature, cos all the upset white boys on that thread that is getting boosted again aren't @'ing us.
It's nice.
@dconley Maybe not? P sure it's been doing that though, cos we've had weird things replying to boosts in the past