That's a bummer, sorry man.

Newly acquired frogs are a bit stressed so having a fully cycled tank (zero ammonia, zero nitrite, and ~.25ppm nitrate) is very important as they acclimate to their new environment. They could also be sick when you buy them as Carlos mentioned. You don't know what conditions the frog was put through before you purchased it, being shipped in a container somewhere and kept in a pet shop puts a lot of strain on an animal. You also have to keep in mind that even slight amounts of ammonia is very toxic to these little guys and that may have been enough to push an already stressed frog over the edge.

This is just a guess but I think these two factors may have caused this to happen.