Most common cause is diet. Too much sugar. Never should have fruit except as a rare treat to impress your friends if you want your tortoise to come running over! An unusually high concentration of cactus, succulents and even dandelion can also cause loose stools.
The other consideration is parasite load. Especially a high load of flagellates. If diet is good with mostly high fiber and 5 days later still loose stools, take a fecal sample to a reptile vet.