Swiss news portal 20 Minuten quotes an eye witness, a military pilot, who witnessed the aircraft shortly before crash. He claims the plane was in a left turn, when it suddenly pitched left forward, which reminded him of a manoeuvre in case of an engine failure. After that he heard an engine rev up and the plane assumed normal flight again.
I live close to the Ju-Air base in Dübendorf and used to see them on approach. I always wanted to take a flight on them, but I never came to it. It may remain so I'm afraid.