That wasn't the case a few years back. Attack any of the wing members, they all attacked you. Which is a logical thing for a wing, flying as one combat unit, to do. There is no point in having NPC wings if they don't offer a significantly more difficult combat experience.The two wingmen in a wing of three fight back after the boss goes down, which is fine as a possible behaviour pattern
Hence, wings are bugged. And I haven't observed any different behaviours to them, either. There seems to be just 3 distinct "rules", none of which seems intentional, especially the 2nd one:
1. If you attack the leader, the leader and only the leader will aggro. Sometimes with a delay if they're hatchbreaking a miner, but they will.
2. If you attack a wingman, no-one in the wing aggros, including the wingman you're shooting at.
3. Wingmen attack you only and only if the leader dies. And even then they have a good chance of just high waking out.
There are also cases where NPC-s just sit there in one place doing nothing, but it's not limited to wings and seems to at least in some cases have to do with their collector limpet having picked up something that the NPC ship can't collect.