Why would my P60 drop wrist pins in sleeve?

My 1993 P 60 dropped 2 of the 3 wrist pins on to the top and bottom cylinder sleeves resulting in scores about 0.015 on the sleeves where the gudgeon pins had run up and down the bores. The compession on the cylinders was 115, 115 110 psi. Yes the circlips were missing which would obviously cause this but why would they have come out? I know the motor has been apart in the past and asume the previous owner that did it forgot the circlips but as the motor is on the bench after a 0.040" rebore is there any other possible cause? I can find no worn bearings but intend to replace the wrist rollers anyway. The motor ran lumpy at idle before rebore but not noticably horendous and was fine by about 2000 rpm.

I just dont want to miss a probable cause while it is apart,

[boats.net]

My quess would be the clips not installed properly or not installed at all.