Natwest's initial response was what you said: his accounts fell below a certain threshold after he paid off his mortgage. Then Farage sought info from Natwest using "subject access request". In those documents, it is "found that an internal committee had deemed his views did not align with the bank's own. This formed part of the basis for cutting him, the document showed, alongside commercial considerations." [1]
[1] https://www.reuters.com/business/finance/natwest-meets-quart...