Occam's Razor suggests that the people who voted for Brexit didn't think that they were receiving net benefits from the EU. I'm pretty sure the net financial transfer is Britain sends more money to the EU than they get back, so it is easy to see why a tax-paying voter might feel that way.

There is an argument that this view is too simplistic and that the trade situation is complicated; but it seems a lot more plausible than a Russian conspiracy to ... do something.

There is always some level of background interference by foreign nations (as an Australian I shudder to think how much influence the US must have over Australia's politics in practice) but typically it would be cheaper to buy off or otherwise influence entrenched politicians rather than try to influence popular votes. I expect the Russians can figure that out too.