A therapy plot is a variation on the "idiot plot", which is a term to describe a story "kept in motion solely by virtue of the fact that everybody involved is an idiot," and that could be quickly solved if the characters were just a bit brighter.
A therapy plot is the same kind of idea, but where the story is purely kept in motion by the fact that every character desperately needs to see a therapist.
"Wow, this book is such a therapy plot. Why are none of these characters seeing a professional?"