A319
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft
We were given a clearance to descend via the arrival into ZZZ and were at FL370. We also were instructed to maintain 270 kts until ZZZZZ intersection and then resume published speeds. 6;000 ft had been put into the altitude window for the bottom altitude of the profile descent and DES was indicated on the FMA for a managed descent.  Aircraft was being operated with autopilot 2 engaged.  FO was PF and is a new hire.  We were both looking at the FO's MCDU as I was explaining how to put the 270 knot restriction on the DES page.  We both felt the plane abruptly pitch up and begin a zoom climb and depart FL370.  I looked at all the instruments to determine why the aircraft started the climb; initially believing that a protection had been activated or we had suffered an undue activation of alpha protection malfunction; but neither was the case. AP never disengaged on its own. AP was disconnected and aircraft was leveled at about 38;800 ft then a descent was begun.  I advised ATC of the event.  AP 2 was then reengaged.  It appeared that the aircraft was starting to climb again so AP 2 was again disconnected.  We engaged AP 1 and resumed normal operations on that autopilot.
[Report narrative contained no additional information.]
A319 flight crew reported an abrupt; uncommanded pitch up and climb when the FMS was set up for a Managed Descent using Autopilot Number Two. Normal operations were resumed with the use of Autopilot One.