It's called cecal dysbiosis, or intermittent soft stool or cecotropes. It's caused by an overgrowth of bad bacteria. In most cases it is caused by the diet, though in a few cases it can be caused by parasites or severe bacterial overgrowth of specific bacteria, and in these cases, immediate vet attention is needed. Usually if this is the problem the symptoms will be more severe and there will be other symptoms like weight loss, lethargy, lack of appetite, though this isn't always the case. If this is a concern, you should have your rabbit examined by your vet and have a fecal test done.
If you think it is due to diet a few things that can cause it are sudden increase or introduction of a new food, a sensitivity to a food or veggie, or too many sugars and carbs in the diet and/or not enough fiber from hay. Too many sugars and carbs is the most common cause of the mushy cecal poop.
The best way to get this to clear up is to feed unlimited grass hay only. This is timothy, orchard, bermuda, or any other grass hay variety. Though not usually oat hay as it can sometimes cause issues if there are seed heads in it. You do need to ensure your rabbit is eating the hay really well. Usually you will see improvement within a few days, though in more severe cases it can sometimes take several weeks before you see improvement. Once there has been no sign of mushy poop for a week, then you can start reintroducing other foods. You should only introduce one food at a time and do it slowly in small amounts, to give your rabbits digestive system time to adjust to the new food. And introducing one food at a time, will help you to know if there is a problem with a particular food. Increase the amounts slowly each day. This can sometimes take a couple weeks to a month, to finish reintroducing the pellets. You will probably not be able to feed the same amounts as before, as pellets are often one of the causes of the mushy poop. At the first sign of soft poop, cut the amount down slightly and see if that clears it back up. It's best to avoid giving sugary starchy foods to rabbits with this kind of problem. That means no grain foods, sugary treats, fruit, or starchy veggies like carrots.
http://www.sawneeanimalclinic.com/downloads/chronic_intermittent_diarrhea_in_rabbits.pdf