Betrayal
It hadn't been expected, and that is why it had shocked Sam so terribly.
The man had been his bodyguard for years. He was a brave, determined soldier, a person Sam trusted, had considered a friend, and he had betrayed him, handed him over to the Decepticons without so much as a "By your leave."
Sam had survived; his friends had torn apart the building, tracking his very faint life signal, to rescue him. After a few days of Ratchet's tender loving care, he had been back to his old self.
Still, the treachery had shaken everyone, the Bots and Mikeala hadn't let him out of their sight for weeks. Sam, for his part, had made a point to refuse any protection from human sources, unless it was Will or Epps, for a very long time.
Sometimes he wondered what that said about him, as a human.