Observers & Time
In 411, December finds out that September didn’t get rid of the traces left of Peter in this new timeline. So, I’m assuming the mini-Observer-conference that will take place in 414 from the promo will see him declare September a rogue!Observer and order his execution. Someone (probably the Observer assassin that Peter didn’t kill in this timeline) then shoots September, but he survives and appears to Olivia in 408 to warn her about dying in all possible futures. Then he disappears from the opera house to appear in Walter’s lab in 414 again to warn him to stick with Olivia, and to hopefully be saved by the Bishop genius.
Maybe he always knew that one day his life will depend on Walter/Peter and that’s why, in at least one universe/timeline, he saved them from Reiden Lake.