I suspect a lot of gentle nipping, fabric bunching, and digging behaviors are the result of a rabbit exploring the various substances and textures they are surrounded with.
I mean, the only tools they have to interact with are those digging and running paws, and their lips.
I've also noticed males that were allowed in the colony pen outside or even in a lawn tractor cage dig differently than the females do. They make what looks to me like a scrape, where they will tear away the surface (annoyingly, just the plants, to expose dirt) and then spray and mark around the dug up area.
So, when a boy starts digging inside...I usually return them to their pen....just in case.