Jesse's finally going after Walt! There are a few things that could have been the tipping point: finding out about Jane, getting confirmation about Mike. How did the writers decide on Brock?
There was a lot of talk about the metric of "Whats the Worst Revelation? Mike, Brock, or Jane?" The general feeling is that Jane is the worst revelation, and it feels like Jesse at this point has figured out what happened to Mike, but Brock was always something we wanted Jesse to find out. That was something wed been talking about since Walt poisoned him. It was just a matter of placing it. When is the worst possible time this could be realized? How about the time when Jesse is actually, maybe going to be okay? Lets do it then and ruin everything!