Poor quality control during manufacturing. If a lifter is machined with just a tiny bit too much clearance between the parts, it will collapse when the engine is shut down with a valve open due to the tension of the valve spring bottoming out the plunger of the lifter, which then takes a long time to refill. If I take the plunger of a properly-machined lifter out and hold it in my hand for a couple of minutes, it will expand enough due to body heat that it won't go back into the hole it came out of. The proper sliding fit between those pieces is around .0001"-.0002". That's a strand of hair split into 5 to 10 equal slices.
Jerry
