Pure speculation: the pleco comes from an environment where frogs and toads get big enough to be a threat to eggs and young, so he doesn't want the frog around. He wouldn't know that this frog stays small. Or, the frog has a thick slime coat, and the pleco is tasting the frog--not good.
Really, that's about the best I can come up with, but it is specualtion. I tried keeping bristlenose in with my ACF, and she killed them all within a few days. I've no idea why, since she did not eat them, and has never bothered the other fish. They might have been trying to suck on her, since she has a thick slime coat. You'll want to make sure this is not the case--a large pleco could easily cause serious harm to an ADF.