This is a complicated topic, but a short and overly basic version:
There are many people in the UK - as in the US, as in most of the West, actually - for whom neoliberalism has not been kind.
They see immigrants getting jobs and struggle to make ends meet and have been told via various right wing media outlets (in the UK, principally the Daily Mail, Daily Express, Sun, Telegraph and Times newspapers), that there are two causes: uncontrolled freedom of movement in Europe and the political classes that are taking their marching orders from an unelected European Commission.
This view misses out important details. Firstly, it's not accurate in the slightest. It also doesn't reflect the fact the sort of people who are the root cause are the rich globalists who don't pay tax, such as people who own the Daily Mail, Daily Express, Sun, Telegraph and Times newspapers, for example.
However, the view has been built up over entire lifetimes. There are sprinklings of truth about sovereignty (akin to the fact that States have limited powers in the US), inefficiencies in how the European Union does its business (c.f. the Strasbourg/Brussels move and split), and so on.
However the UK gets far more economic benefit than it spends, and the poorest of communities like South Wales have had far more spent on it in targeted social programmes by the EU than has ever been spent by Westminster.
Let me give you a direct example: Lincolnshire is a farming community that had seen declining populations for decades. A large number of farming labour jobs have been filled by people from Europe - particularly Eastern Europe - because they are prepared to do work locals are not, at a price that locals will not consider.
Lincolnshire is a hard Leave area. Boston is IIRC the highest Leave vote in the country. When you ask there people why, one of their favourite anecdotes is the strain the Europeans put on local services.
They sit in an A&E unit waiting hours to be seen, and are surrounded by Eastern Europeans. They think "all these people here are the reason my waiting time is so long, they're using services I pay for! If we got rid of them, I'd have been seen by now!"
The context they are missing is that multiple A&E units in that county were slated for closure. The reason they remain open is because of high demand - from Eastern Europeans.
If we block freedom of movement, they would not be seen quicker. The unit would not exist. It would be closed. But that's not how they see it.
The real culprit as to why waiting times are so high is not because of the Eastern Europeans waiting with them, but because the government has cut funding, because tax receipts are lower than they could be, because certain rich businessmen have lobbied for tax breaks and then told the editors of the papers they own to blame the foreigners...
So there is a huge amount of anger built out of lies and misdirection that will take generations to undo. And they are so convinced of these lies, they are prepared to cause chaos. One of them before the last referendum murdered an MP (a woman called Jo Cox), and that will be but the first of many.