The Doctor shrugs a little. "Your being here specifically... definitely improbable. But I doubt the event of your existence is what he coded into it. Point of no return sort of thing, more likely."

And he did go after one of Jack's team. If there's anything Jack would have been concerned about when it came to backsliding, becoming a threat to them would have been high on the list. He pointedly doesn't glance at Tosh - the Doctor much prefers when Thane's attention is on him.

"I can't tell you that, though, because I honestly don't know. He's difficult to predict at the best of times, and the time just before you arrived, the time when he wrote that note... was not that."