Umbilical hernias can be hereditary. But they also can be trauma induced. An umbilical hernia is an opening in the abdominal wall- if some of the intestines pass thru they can get strangulated in the “hole” and that will cause the intestinal tissue to die.
Injuries such as a hit by car can create a tear in the abdominal wall- even without an external injury in that spot. I saw this in a cat once at the clinic where I work
I suppose with rabbits it would be very hard to find the typical “bulge” that indicates a hernia- as they are typically located on the abdomen- but could be on the rabbits side below ribs. Most buns are not too thrilled with a belly rub- combine that with thick fur and a small hernia woud not be glaringly obvious.
So sorry to hear about your loss- thanks for sharing. I will be sure to have my vet specifically palpate my bunnies abdomen when she is spayed in a couple of months. If she is asleep- they should be able to feel any gaps in the abdominal wall if they are there. I had not even thought of this oreviously.