I don't think the ship was originally rigged to explode, rather, any of the preds with their wrist-comm could order it to self-destruct or program the destination.
Frankly, as someone with some little military experience, I cringed at how these 'Special Ops/Heavy Hitters' did some stuff in the movie. Bunching up together when contact was expected, shooting from the hip (I mean, besides the chick sniper and yakuza with the peashooter, notice how none of them AIMED with their weapon sights during the 'dog' attack? The SMG's, AA12, AK etc were all below chest level), walking out in the open (the rock pool part esp.) etc etc. OK ok its a movie...
BTW, did the Gatling (I cant see, was it a GSh or an XM/M?) and the AA12 do any damage at all in the entire show? I know the pistol rounds were bouncing off the Pred's shield/armour and the Sniper Rifle (what model was it anyway?) could hurt them. I suppose the Pred's shield/armour can resist up to about assault rifle rounds then?