Of course this happens while Catalina is in the middle of actually making a sensible suggestion.
Given that Rhoda has awakened, it really shouldn't be too shocking that she has a spell like this. I mean, it might be a little surprising that she has another spell, but it being something like this is a fairly Rhoda thing to have.
This is an example of a scenario in which there's a sound effect only because it felt off without one. Really, this transformation should be silent (because magic), but the panel didn't feel complete without some manner of "poof" dealie.