Hathaway gonna Hathaway.

EDIT: Let's look at the bright side - the NCR isn't going to be holding the bridges either. And I do believe the mission failure points specified it would be a failure if the bridges fell to the NCR. They aren't falling to the NCR - they are falling to a couple of artillery shells, into the river. Hathaway may have had his own, disloyal reasons for his actions, but he may have just stopped a full-blown NCR invasion. At least for a while.