Perhaps it was something to do with a tradition of naming after kin, and with families often being closely inter-related? If they all lived in a small community, perhaps it's possible the mother had an uncle or brother with 'Crompton' as his first name? (Yeah I know, another guess).

Families higher up the social scale seem to have been using maternal-kindred surnames as first names from even earlier in time. There's an anecdote about a 17th c. gent called 'Bulstrode Whitlock', whose mother's maiden name was Elizabeth Bulstrode. At the christening his godfather had the privilege of choosing the baby's name, and said he was absolutely determined to honour the child's mother in so doing - so they could either name the boy 'Elizabeth' or 'Bulstrode', it was all the same to him. They chose Bulstrode.