Boris was promising Gove a "senior" cabinet role in return for his backing, rumours are either Chancellor or more likely, Home Sec.
Gove wanted something absolute and nailed down, not some kind of airy promise as Boris is bricke on detail (don't think anyone thought Boris would screw him intentionally but has been known to "double count" on offers from time to time). Boris procrastinated, Gove's wife pushed him into a corner, Boris continued to procrastinate so Gove stood without any pre-warning for Boris. Boris knew nothing of this - apparently spent all night with strategists ready to go.
Without Gove, Boris has no senior MPs backing him and no base to stand on. He wanted to run but Gove pulled the rug out from under.
Sauce: Friend who went to school with Comrade Osborne.