That is exactly what is happening. I remember this issue happening before and we were able to rename the behavior slightly to not invoke the ADDT behavior. You don't, by chance, remember what it was that those of us with ADDT installed needed to alter, do you?
I will scour the forum to see if there is any information left on it.
I also found out that it only happens on the Server Behaviors menu - so there is a slight work-around where if you flip to the binding tab and then select file manipulation from that menu, it will actually add the webassist delete file behavior just fine.