Obviously, because Brexit was just fueled by lies and propaganda spread on Meta platforms and twitter to fuel enough anti-european sentiment to leave the Union. They appealed to their sense of "greatness" with disinformation in order to separate one of the only nuclear powers there and the most militarily prepared. UKIP gets their money from the same place all the far-right groups all over the world do. The Heritage Foundation. That's what all the talk about invading Canada and Greenland is about, split NATO forces so there's less support for Ukraine.
It is considered widely as one of Putins biggest successes in driving a wedge into Europe. Remember how much money of the "Leave" campaign came from Russian sources.
The Leave campaigns were largely funded with Russian money, and there were massive pro-brexit disinformation campaigns by Russia on social media. So, not caused by, but massively aided by Russians. Very much like Trump - both projects designed to cause division in Western countries.
hard to ignore trumps speeches include alot of russian/putin talking points and buzzwords. remember trump immediately fired comey after he won, comey thought trump would stand against russia, did he not notice he was always standing with him.
David Cameron caused it by adding it to the Conservative party manifesto to help them get reelected.
I’d be surprised if Russia didn’t at least attempt to influence the referendum in favour of Brexit. They have been interfering in elections for years and Brexit would be beneficial to them as it made both the UK and the EU weaker.