If your rabbit is producing normal round fecal balls, and it is the cecotropes that are mushy, the most common cause is a diet with too many carbs/sugars(usually from pellets and treats) and not enough fiber from hay, though occasionally a rabbit can be sensitive to certain veggies or hay.
Usually the reason to see a vet with mushy poop problems is if it's not the cectropes but the fecal poop that is runny, mushy, or odd shaped, which is a much more serious and immediately life threatening health problem. Or if you have already tried changing the diet and it didn't fix the problem, or you suspect there is something going on more than a simple diet issue. There are some health issues like arthritis, obesity, dental problems, liver problems, that can be a cause for mushy poop on the bum.
To correct cecal dysbiosis where it is normal round fecal poop and mushy cecotropes, it's recommended to reduce or cut out pellets temporarily(sometimes permanently), and free feed a good quality grass hay(not moldy, no noxious weeds) making sure the bun is eating the hay really well. You do have to be very careful to watch your rabbits hay consumption, and make sure your bun is eating plenty of hay. You don't want a rabbit going more than 12 hours without eating/drinking. If your rabbit won't eat the hay you have, try a different grass hay like orchard, timothy, bermuda, meadow, etc. Key is getting your bun eating lots of hay to help correct the imbalance of micro organisms in the cecum/gut, which is what causes the mushy poop to develop from too many sugars/carbs in the diet. If your bun won't eat the hay even with reduced pellets and trying different types of hay, then it's possible your rabbit could have a dental issue preventing hay eating, so you'll want to have that checked by an experienced rabbit vet.
Here's information on mushy poop from diet issues, and the diet recommendation to help correct it.
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
https://rabbit.org/disorders-of-the-cecum/