I always thought that Selphie's team drove the BGH251F2 to Fisherman's Horizon. Obviously, they didn't know the other team was at Fisherman's Horizon at the time, but I'm guessing they thought they could get to Balamb if they crossed the bridge.

Anyway, the other party, upon seeing it, decided it was an enemy and attacked, thus making it not a good time for Selphie's party to reveal themselves.

As for the tank attacking, it was malfunctioning. It had previously blown four gaskets, and taken a lot of damage. So, much like the T-1000 at the end of Terminator 2, it was malfunctioning, and thus attacking without being told to. For all we know, it had been attacking all the way from the missile base.