This was briefly explained in the Times article I linked to (
https://www.thetimes.co.uk/edition/...eged-on-promise-to-help-theresa-may-kq0t5fktn ): it would seem that many British Parliament Members cannot stand the idea of Britain "trapped" in a legally binding customs union with the European Union indefinitely (i.e. with no time limit, because of the Irish backstop).
This point of view is perhaps to some extent understandable (I mean, "not crazy", and not "nebulous"). Indeed, what is the point of leaving the EU if the UK still depends on some EU rules, it has no longer control on? Can you imagine democratically elected MPs transforming the
once proud UK into some kind of European colony "for ever"? It seems to me that a possible solution to this concern (though I do not consider myself to be an expert on these issues) would be to limit this mandatory Customs Union to a maximum of five years (if it ever happens, see the internet and Google for details).