He might have made a gamble for political reasons. But, don't forget that there had been strong demand for a referendum on the matter since at least the mid '90s. It was long overdue, and (being cynical) had been denied because the outcome wasn't going to be what the government wanted (i.e. they knew the majority would have consistently voted leave, and didn't want to hold the referendum until they could guarantee a remain vote).

When given the chance to express their true opinion of EU membership, the majority voted leave. Not a particularly shocking outcome, given the lack of love the EU had in the eyes of most of the populace, and always had despite several decades of indoctrination.