Why does memory (RAM) fail?

[Tony Arnold]

I'm after a bit of insight rather than any solutions as to why memory
fails. I rebuilt my PC about 3 years ago with all components brand new
including 64GB of RAM. A couple of weeks ago I started seeing some
weird behaviour such as system freezing, random rebooting, VMs failing
etc. I ran memtest which showed a large number of errors. I was
surprised to see this given the memory was only 3 years old which, to
me, seems like rather a short life time!

The memory was 2 x 32GB DDR5 UDIMMS from Vengeance. The chips are rated
at 6600mbps whereas the motherboard is rated slightly slower at
6400mbps. Motherboard is a MSI Pro Z790-A. Would the speed difference
cause the memory to fail?

I've now replaced the memory chips with Crucial chips whose speed match
the motherboard's speed. memtest shows zero errors!

Any insight as to why the memory failed after a relatively short time
would be much appreciated.

Regards,
Tony.
-- 
Tony Arnold MBCS, CITP | Retired IT Security Analyst