Yes, I do think that buns crave what is good for them when they are ill, as donnamt says. They'll always crave sugars which are bad for them, like children like sweets.
Grass is the natural food for bunnies, so they are designed to function best on that particular balance of fiber/nutrient. There are over 1000 different species of grass, & both Thumper & wild buns are very selective about which grasses they eat changing from day to day.
Thumper can only chew limited amount before his tongue gets sore with molar spurs. Time & time again I have watched him attempt to get maximum nutrient, fiber & fluid from his food with the least chewing/tongue movement.
Why grass rather than hay? I have no corroboration for my hypothesis at all. The green in grass is chlorophyll. Chlorophyll has an identical molecular structure to haem except that chlorophyll has a central magnesium atom & haem has a central iron atom. Haem is not only needed for haemaglobin to transport oxygen to tissues but is also needed to release the energy from glucose (in the cytochrome oxidase chain). Perhaps with the increased cell mass in cancer, your bunnies are trying to get nutrients in, & reduce waste products, in the most efficient way. It is also possible that they are craving cancer fighting oxidants.
This is pure supposition on my part. My biological sciences are very out of date. I call on those more up to date on the forum to comment on my hypothesis.