To list some things that can cause this:
1. Inadvertnally moving a control while reaching to push the PTO button.
2. Weak springs in control, not bringing it back clsoe to center.
3. Actuator getting worn, not returning to neutral or sensor in actuator worn and not sending correct feedback signal.
4. Control valve spool or the valve centering spring binding not allowing it to center.
Try starting without hands on the controls. If problem still happens, with key off jiggle the controls trying to feel for most neutral spot then re-try starting without touching controls.
If either of these don't work, the best bet is have a dealer hook up his laptop to read the system to see what is going on.