Its any mutable things that could have caused this. Not being there its up to guess work. I have seen some simple but stupid stuff. No lube on the push rod tips (both ends) is one.

But in the 40 or so cams I've installed over the years not one failure. I'd say 80% have been retrofitting a new cam in a well used engine.

But.....

I never run big springs on break in. The lightest spring that will not coil bind.

Oil only on the lifter bores. I have used just a little assembly lube mixed 50/50 with 5W30. Oil is oil on lifters so synthetic is OK if the rings are seated. On the cam lobes I will brush on some assembly lube.

I use a lifter extractor to clamp on to each lifter as I install them and with just oil in the bore, I hand spin it while pushing down to feel for any binding.

I've put flat washers curved to fit under the rocker shafts to lessen the preload on Hyd FT lifters. Loose lash Mech FT's

But with todays cam designs, I do not think I will not ever run a FT cam again. Rollers are getting cheaper and offer up so many plus'es over a FT.