The issue is you’d really need an evolutionary pressure for them to develop that attack behaviour. It could be defensive, but jumping on the thing you’re trying to fight off is a rather bold strategy, especially when the results aren’t instant. Alternatively, it could be an attack behaviour, allowing them to take down larger prey. Thus just leaves the issue of how the frog would consume its meal. They could probably evolve to swallow smaller prey, but the obvious adaption, which I think we’d see in this case, would be for the frogs to evolve teeth.
The issue is you’d really need an evolutionary pressure for them to develop that attack behaviour. It could be defensive, but jumping on the thing you’re trying to fight off is a rather bold strategy, especially when the results aren’t instant. Alternatively, it could be an attack behaviour, allowing them to take down larger prey. Thus just leaves the issue of how the frog would consume its meal. They could probably evolve to swallow smaller prey, but the obvious adaption, which I think we’d see in this case, would be for the frogs to evolve teeth.