How long had the two lived together when that took place?Just a warning coming from my recent personal experience:
Do not be misled by petstore employees when they tell you plecos and african dwarf frogs (and other small aquatic animals, if i'm not mistaken) are compatible tankmates. I have recently witnessed a pleco nearly battering an african dwarf frog to death by attaching its mouth to the frogs body and moving swifty along the substrate in the bottom of the tank. The pleco was completely aggressive in posture and did this repeatedly until I was forced to seperate the species. I have also heard of several other accounts of this with dwarf frogs and other small fish and animals.
Once again, this is just a friendly warning to aquatic hobbyists everywhere. This was awful to witness and had I not been home, I'm sure the frog would have suffered injuries leading to death or drowning, etc.