From the pinned official rules:
Q: Does a Proximity Mine, when dropped overlapping a ship in the System Phase, detonate immediately?
A: Yes. When an object is placed underneath a ship, that ship counts as overlapping that object.
So a ship drops a proximity mine under a group of ships. Does that mean, as I suspect it does, that every ship now overlaping the mine template has to roll for the effect of the mine (1 hit + all hits and crits rolled on the 2 dice)?