Well for any of us that have tossed lifters it's almost always on start up. From a bent pushrod but oil usually holds it in when under pressure. What happens is oils drains away and on start up they have a tendency to toss the lifter up and out!!
I had a cam that was terrible for bending pushrods back in the early eighties. I used to keep 3 or 4 in the trunk most often when you got gas or other start it up and 0 pressure. I'd pop the intake put lifter back in new push rod and go. later noticed the cam lobes were flaking took it back to mopar and they replaced it (I'm amazed too!!!) and never another bent pushrod.