Leopard frogs can live close to 10 years, so unless you captured an old female, something shortened her life. Since we don't know the water chemical status, can't comment on that. Water changes are good, would not replace all the filter's media at same time because doing so will create a mini cycle. Only other thing noted is that from description, your tank is all water set-up and most care articles recommend 1/4 of tank with substrate (3-4 in. of shredded coco) for frog to burrow if desired.

Appears only thing left to do is observe the others for any similar signs, and take frog(s) to veterinary with frog amphib/reptile experience ASAP if those symptoms recur. Good luck !