Most stutterers just begin stuttering. No "trigger" necessary. Sudden onset stuttering exists, though, and should typically be paid attention to in a different manner. And yes, stutter is known to be able to vary a lot. It's one of those things we have to get used to, so we don't keep getting surprised. Why don't you go see a speech therapist again? Maybe you can figure out if there are some things you can do to help your situation.