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
And to hark back to Phil's original comment the operative word is "cold." From what I've seen, it doesn't take much to let the lifters collapse. When the entire engine is 70 or 80 degrees things are different than when the entire engine is 20 or 30 degrees. And there is also the question of what oil? If you're using 10w30, when cold the viscosity is 10 weight. There is a difference between 10 and 30.
