Rabbits are hindgut fermenters similar to horses, and their large cecums are used similar to a rumen for anaerobic microbial digestion of roughages they consume. read more
The phenomenon doesn’t correlate strictly with a fibrous diet (though the pellets would probably not hold their shape otherwise), since animals like cows and buffaloes have a fibrous diet but most definitely have soupy poop. Obviously, the digestive process itself–which is different in ruminants such as cattle–is also a factor. read more