Senior US embassy diplomats in London, backed by the US national security adviser, Jake Sullivan, have directly warned the UK’s Brexit negotiator, Lord Frost, that he will inflame tensions in Northern Ireland if he does not compromise over border checks.Short version: Boris and the gang just wanted any sort of "hard" Brexit and didn't care much about the details and the consequences, figuring they'd bullshit their way through it. Worked well for them their entire lives!
Removing Northern Ireland from the EU causes a bit of a problem, as it replaces the border the EU/Good Friday Agreements had removed. A border that requires, among other things, food and agrictulural checks to be consistent with EU law/treaty. Can't impose that and then pretend it doesn't really exist. It does!
Don't have to have any particular opinions about Ireland/NI to understand that re-imposing a border where there was not one is a problem!