Sometimes even a spayed rabbit will nest.. Or it could be from stress. Some rabbits do NOT like changes in routine or having someone new care for them, no matter how temporarily.
As long as she's spayed (or at least not near any un-neutered males!) and you can't see where she's actually pulling the fur from (most likely chest and belly) or it stops within the next week or so, I wouldn't worry about it.
Sometimes when stressed out they'll also pull fur from each other.. So if you have more than one bun, check the other one as well for any developing thin spots in the fur.
Turdle