Presumably the machine. It bugged me that the machine wouldn't warn them earlier, but then I thought about it more, and the way the machine works is probably the cause. I mean, Simmons probably wasn't calling anyone or discussing killing Carter/Reese, so the machine couldn't identify them as in danger. And since the entire city was looking for him, he was probably avoiding cameras, so the machine knew there was someone who was dangerous out there, but had no idea who he was putting in danger. So then when he surfaced, the machine calculated what he was going to do, but it was too late..