My guess personally is that they have grow tired of all the immigrants that have come in and taken jobs away from Britons.
I don't think there is a single clear picture, Major.
If we break things down to the individual nations involved, the most decisive result was in Scotland which voted to remain. Northern Ireland also voted to remain and Wales and, a bit more strongly so, England, voted to leave.
A notable exception in England was London who probably have one of the highest percentages of immigrants in the UK but voted to remain. That said, I would think immigration was considered an issue by quite a few in other parts of the country...
Yet another theory is the "little to loose" one, ie. those working with mortgages and on reasonable middle incomes were more likely to vote remain than those in rented property and low income. Tied in with that one can be the idea that some leave voters did so more as a protest against the governments than anything else.
Then there are the people who simply do not trust or don't want to be what they perceive as being ruled by Europe... And so we could continue to go back and forth on this and pick up different theories and opinions depending on which papers we read...
Whatever, overall 52% of the UK as a whole voted to leave and we have to look at what happens next. I don't think it will happen but there is talk of a second referendum over Scottish independence, presumably with an independent Scotland looking to joining the EU. I suppose even less likely could be a move in Northern Ireland to join the Republic of Ireland (although I can't see the Unionists going for that one). As I said I think any of this is improbable but it is not the full 100% clear to me at the moment that the UK will continue for much longer in its present form.
On other things, I think things will be tough in the short term with the effects of Brexit being felt more strongly next year. The longer term is probably anyone's guess with much depending on whether we can still get access to the EU trading system and what other trading deals may be struck.
At least that would be my forecast if "everything else" stays the same. The whole world strikes me as being pretty volatile at the moment...