B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 132; Flight Crew Total 10908; Flight Crew Type 7020
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control
Aircraft
In cruise flight at FL380 near the beginning of the OZZZI 1 RNAV arrival into ATL; told to descend to FL350. PM (Pilot Monitoring) set altitude selector to 35000 ft. and I pressed altitude intervene button. We descended to FL350 and were at that altitude for several minutes when the aircraft began to descend near WLLSN waypoint. There was a moment of disbelief and why is it descending and how is it descending when 35000 ft. is in the altitude window. I disengaged the autopilot and auto throttle to take control of the aircraft and climb back to FL350. Lowest altitude descended to was 34;600 ft.  After reestablishing autopilot and auto throttles back on at FL350 we tried to assess what had happened. Our top of descent point wasn't for another 80 to 100 nm; 35000 was in the altitude window and we were in VNAV path. We could not determine why the autopilot descended the aircraft at that time.
Aircraft descended from cruise altitude for no apparent reason. At FL350 approximately 30nm North of WILLSN intersection. Aircraft began to descend with no known input from crew. LNAV/VNAV engaged; FL350 in CRZ page and Mode Control Panel altitude window. Only indication was sound pitch change and aircraft movement. PF disengaged AP (Autopilot) and AT (Auto throttle); hand flew to correct. Bottomed out at approximately 34;600 ft. Once back at FL350 we rest Mode Control Panel; engaged AP/AT and aircraft performed normally remainder of flight. I have no idea why this occurred. All seemed normal before and after incident.
B737 flight crew reported excursion for level assigned altitude with autopilot engaged for no apparent reason.