I almost had the masks drop on a B767 several years ago. We were on descent through about 37000' when we got a cabin altitude warning. There was no rush of air or anything dramatic so we initially thought it was false until I saw the cabin was climbing through 10000', at about 500 fpm with the outflow valves closed in auto.
I noticed the bleed pressure was quite low, probably because the high stage bleed valves had stuck closed on both engines. Our solution was to extend the speed-brakes and increase thrust to get the pressure high enough for the cabin to descend again before the masks deployed.