You haven't hit a base, you've crashed in the middle of nowhere in Tau held territory. The aircraft you were on was some kind a large transport plane, large enough to carry at least a company's worth of troops, equipment and vehicles (I don't know if such a vehicle exists in the fluff, if not just assume it's native to Ferin). The reason for all the different people from different branches of the military on board varies somewhat from person to person, but the fluff does mention that regiments are split up for combined arms purposes, so here's a few ideas:
McTavish's squad was sent for a special mission, that he hasn't been briefed on yet.
Wexler's company has suffered sever casualties, and has didn't need a full transport.
Note's company is a small, veteran force from the Red Talon's homworld (Iron Hands successors). The company is small enough that they can share a transport with the Catachans.
Drakoon is a techpriest, and was probably travelling alone for separate reasons.
Vyper's Ogryn (name pending - maybe he forgot?) probably got on the wrong transport by mistake.