Why a group? I suspect it’s because when we’ve seen this used as a tactic it requires the whole defense to play along. Otherwise, if only 1 or 2 players engage and the rest stand twiddling their respective thumbs then either they all get trampled, or they step back far enough and get pinged under 18.38 for leaving the lineout, or they step out the way expecting a PK and watch bemused as the huddle - who are switched on enough to keep the BC at the front as a meat shield - walks the rest of the way to the in-goal for a try.
*edit to clarify the BC must stay at the front to be open to a tackle.