Center Hydraulic Pump cavitation
I can access our MEL on-line now. There is a note at the bottom of the MEL for this item that says:
"Note: Flight tests indicate that the center hydraulic pumps would start to cavitate at flight levels above 270."
So ftrplt is essentially correct in that apparently the APU loses enough pneumatic capability by FL270 that it is incapable of providing sufficient center hydraulic head pressure to keep the center pumps operating normally.