I agree that it may be set in step #3 of 4 step buttonholes. Do you have an instruction book so you can pretend to make a buttonhole? This happened with the Elna once. My daughter didn't finish up a buttonhole and when I started to sew, the machine ran backwards in ZZ. Took me a few panicky minutes to figure that one out.
My favorite repairman is working part time at the LQS. Every time I see he's there, I wonder if the Elna needs anything - she doesn't, but it's good to know he's around again. He had the Singer store for a while. I should have look at this T&S before I start dismantling it.