We've had a couple of similar episodes with out new Raymarine ST6000 setup... no plotter, just auto.
What I noticed is that in addition to the offest the heading scrolled the WRONG WAY.. ie a turn to starboard generated a lower heading (in degrees). Since this represents positive feedback for the controller, the auto just layed the helm over hard and stayed there.
It's happened maybe 2 times in the year since I bought it, and a 'reboot' cleared it each time. A pain, though.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"