Steve, TH didn't get a lot of exposure from McLaren the last years. He was asking and McLaren weren't giving. McLaren is pretty focused on their roadcars had a dire first season with (Japanese) Honda and are not exactly tipped for pole any time soon. Biver is managing a turn around and wants results/exposure. Possibly add incompatibility of personalities, and the time is ripe for change. Voila! A nameless engine is available at RBR. Which will guarantee than F1 broadcasters will mention TAG more often than Rolex. Possibly lucky in his timing but he swooped at the chance.

Ps: A more critical approach to his marketing of the Connected Thing can be read at engadget or techcrunch.