Statistically it doesn't seem likely that Monzo and its ilk would be so overrepresented merely because their customers are more fazed than others about having their money taken away. Still, if the FCA have published meaningful statistics I could be proved wrong.
https://www.which.co.uk/news/2021/09/why-banks-are-freezing-...
This in turn means that they will have their risk assessments inverted from the usual high-street banks: optimise signup/account creation flow, and deal with AML requirements in a slightly delayed fashion. Making it really easy and smooth to open a current account brings in a surprising fraction of the crowd who would be rejected or otherwise earmarked by high-street banks.
Being digital upstarts, these modern banks also don't have the fraud and risk departments their established competition has. In order to not get hammered by the FCA, they almost certainly veer on the blunt instrument side when dealing with suspicious activity. And law of large numbers guarantees that there will be a significant number of false positives.
This, I should add, at a time when there was a high incidence of fraud with people trying to open business accounts for dormant and non-existent companies simply so they could claim Govt backed loans where it was clear that very few checks were actually being carried out.