This is the second time this has happened in the past 24 hrs playing Survival 1.0. It's happened both times right after a Zirax attack. The last time I went out to loot a crashed drone and as I came back to the base both of my Projectile Guns opened up on me. Is this a known bug or am I missing something?
My problem comes from the fact that the core was intact when I went to loot the drones after my guns shot them down, So there were no Zirax to left to destroy the core. If a stray trooper got into the base the internal sentry guns would have finished them off. However it appears that the Zirax can remote hack a core if they're withing 10 meters of it, so I moved the Core deep into the mountain and have no problems since. Well, none that my guns couldn't take care of.