It did happen quite quickly so I can't say if the pitch flew away from the FD bars and then the FD changed to G/S* or if the FD mode changed by itself to V/S to climb towards the G/S.
I strongly suspect it was the first case though. Maybe the APPR mode captured the G/S too far away and commanded the airplane to leave the present pitch attitude disregarding the FD bars. What I can clearly recall - other than the usual "WTF!" - was the altitude alarm which, if my memory doesn't fail me, gets trigger when the airplane drifts away 250' or more from the altitude maintained.
Bright side: it woke me up in time for the landing!