One explanation could be that she’s producing excess cecotropes (cecotropes or cecals are the proper names for this type of poop) or too mushy cecotropes. Buns only eat what they need of their cecotropes and they can even refrain from eating them if they’re too mushy. Another possibility is that she’s over-weight and/or old (osteo-arthritic) and has trouble reaching the cecotropes. Buns prefer eating their cecotropes right when they come out. There is a nerve-signal when they come out so the bun knows when to reach down and eat them. In old rabbits nerve impulses from the rear can become slow or weak, and then the bun might not know there are cecotropes.
Cecotropes have an extremely sticky surface and when they get mushed they stick to the buns fur and you have to either comb them out with the aid of cornstarch (to wick up remaining sticky moisture) or give the bun a butt bath. In some cases a bun can need a sanitary shave downstairs, but that’s best done by a vet or vet tech that has the proper equipment.
The main cause for mushy/excess cecals is diet. If you have introduced a new type of veg, that could be the culprit. If she’s getting too many treats, that should be the first thing to cut out. You can also try to cut back on pellets (don’t cut them out entirely) to encourage the bun to eat more hay. Hay is the best thing for a disrupted bunny gut flora.
Here is an article about soft cecotropes, also known as intermittent soft cecotropes, ISC.
http://rabbit.org/intermittent-soft-cecotropes-in-rabbits/