It can be caused by a tiny cut or scratch too which bacteria can enter. Not sure about the heredity part since bumble is a staff infection. That's a shame it has been so bad in your Muscovy's have you had any other poultry to have this problem?
I agree here, any animal that stands in mud or soil that has animal feces in it, with an open wound on the foot, will get a bumble.
It happens, and often hard to avoid it.
Injuries to the feet can be from rocks or jumping down and landing on a pebble.
Once the bacteria is in there, the bumble plug begins, and it is a nightmare to cure, especially in water fowl.
I have raised Muscovy here and had no issue of them ever getting bumbles.