A330 goes from MACH to SPEED mode when changing flight levels
The other day during the cruise ATC descends us from FL380 to FL340. Flying in managed speed, at mach 0.81, the autopilot modes before the descent are MACH ALT CRZ NAV. We descent in V/S -1000, and for some reason shortly after starting the descent the speed mode changes from MACH to SPEED. When leveling off at FL340 The speed mode went back to MACH.
I can't think of any reason why the speed mode would change from MACH to SPEED at such an altitude. Usually in the climb it changes from SPEED to MACH around FL280. Even though we were still in the cruise phase, the speed selected in PERF DESCENT was .81/300.
Can anybody shed some light on the reason behind this mode change?