WhatEVER the curse was; it appears that extra-biblical texts are used to justify the reasoning.
Or self serving speculation based on a plausible etymology for a name in this case. My own first reaction as a new believer when I learned about this (it was in personal conversation with a Methodist preacher who was rightly dubious about the idea) was “OK, suppose there had been a curse; is it the duty of Christians to add to it?”