Wouldn't discuss too much about it, same happened to me two days ago in a very normal situation. Descending with my 737 NG in VNAV and LNAV mode, normal operation, on speed, on track, on path - und suddenly and unmotivated AP disconnected, btw with sounding alarm. Re-engaged, normal operation again, didn't disconnect again. Reported to the maintenance on ground, they'll go check but didn't had any idea why it could happen.
Remember, an autopilot might be female, they sometimes misbehave unmotivated...
Or maybe she (AP A) just didn't like the frenchmen, as it happenend soon after reaching french airspace?