That's really great news that you were able to provide her with a home, now I KNOW she's a very lucky girl!

Well if she digs when you are on the floor and she can hop away, my next guess is that it might be a nesting thing...
females especially love to dig (and tear up things too).

Has she been spayed? You need a bunny specialist vet because not all vets know how tricky anesthesia can be for bunnies....they are very sensitive and the anesthesia can kill them easily.

Females are especially prone to reproductive cancers. Also spaying will make her urine less strong and she will become more loving and placid. That's the only other thing I can think of as a cause for the digging.......I suspect this is just one of the great mysteries of bunny life.

p.s. By the way I notice you said she gets lettuce....absolutely no iceberg lettuce...too high in water and no vitamins, etc.....iceberg will give tje bun a soft stool and eventually diahrea, which can kill them if left untreated. So Romaine, green or red leaf lettuces, swiss chard, collards, little bit of kale (also high in calcium), outer leaves of cabbage not the inner green pale ones, dandelions, etc.