An ERJ 170 Captain reported hearing annunciations sounding while in the forward lavatory during a break. The Captain returned to the flight deck and was told that 8 or 10 EICAS Messages had populated including a MAU1 FAIL. The flight crew were able to engage the auto pilot and stabilize the aircraft 1;000 feet below assigned altitude. The flight returned to assigned altitude and continued the flight to destination airport.

Narrative

During cruise flight I had to use the lavatory. I went to the lavatory. When I stepped outside of the lavatory I heard annunciations going off in the flight deck as well as the pitch attitude going up in the plane. I told the FA I was ready to back in the flight deck. I got in the flight deck and sat down. The FO seemed a little flustered and I asked what was going on. I looked at the EICAS (Engine Indicating and Crew Alerting System) and the only message was 'YD OFF'. Then I noticed the autopilot and auto throttles were off and he was hand flying the aircraft. We were at 32;200 feet and descending; as our assigned altitude was 32;000 feet. Name said that there was 8 or so EICAS messages and he believed the top one was a MAU (Modular Avionic Unit) failure. Since there was no message for any failures anymore I told him I was reengaging the autopilot and autothrottles. I did so; and they worked normally. Then I asked why we were descending and he said that he got off his altitude while hand flying. I asked by how much and he said 'I think 1;000 feet'. I only saw a 200 foot deviation by the time I got in the flight deck. He said that he told ATC but I wanted to make sure. So I got on the radio and told ATC we had our autopilot kick off and we are going back to FL320; ATC responded by saying no problem. Then I was trying to figure out what happened; since I was in the lavatory when all of the EICAS messages came on. The FO said he does not remember what the messages were; but that he saw a 'STICK SHAKER' message. I believe this was because he inadvertently pitched up the aircraft when the autopilot kicked off. I only saw YD OFF and that message went away when I reengaged the autopilot. The rest of the flight went well without any messages. I called maintenance on the ground and asked if they could figure out what message popped up. They were not able to tell. Since I was not in the flight deck when everything happened; I'm not quite sure what did happen. For the FO; I think he could have watched the altitude more closely to not deviate from altitude.