You remember how I told you each one of us got an A.I.? The A.I. Tex got was Omega, The Alpha's anger and rage. Omega had this weird ability that let him jump from suit to suit, so when Tex showed up at Blood Gulch, apparently that's what Omega did. I don't know the details, but he "infected" Caboose for a bit and that's probably why he's the way he is now. So when Tex left Blood Gulch and crash landed at Outpost 17b, Omega started infecting the sim soldiers there. That is, until The Meta showed up and took him. And Tex. Probably.