Sitting turned off for a while, the filament gets cold, and its resistance decreases to a pretty low value. When the equipment is turned on cold, the current sees a low-resistance path through the filament circuit and charges ahead, heating the filament in a matter of a second or two and raising its resistance to where it's designed to be. It sounds brutal, but unless you have something like the Harbach soft start system to bring the filament voltage up in steps, it's gonna happen. Apparently, the tubes are, and have been for decades, able to withstand the sudden application of full filament voltage with no harm done.