I bet the turn was last minute, and Ambrose approached Vince, Reigns, and Rollins and told them he wanted all the smoke, and asked if they were ok with it. They were going to turn him anyway somewhere down the line. Dean's in this PG era of the E but he was ushered by the Attitude era. He has an OG sense of mind. At the same time i think he's aware. He knew pushing his heel turn back a few months or so would get the same reaction he would get, before Reigns' announcement. He would possibly get cheered.
This was the time (after Reigns' announcement) to get not only the maximum reactions from the audience (because reactions are the most important to the E now days), but the reaction they want. He is a heel, who will get boo'd. Which is as of late, been damn near rare to see. Most were in shock tonight. I expect nuclear heat next Monday. Like someone said earlier in the thread, this SHOULD establish him as the top heel.