<VV> Strange (?) double failure

[AeroNed at aol.com]

I thought this was somewhat strange and I thought you might be  interested...
 
About 5 years ago I rebuilt the front end in my coupe, new everything. I  was 
getting it ready for an autocross tomorrow and was backing it out of the  
garage when BOOM. The driver's side lower ball joint nut popped off and the ball  
joint shaft came out of the steering knuckle. Fortunately for me, the threads 
in  the nut were the one's that stripped out. There was no sign of the cotter 
key.  Replaced the nut and cotter key and all is well.
 
I decided to check all of the other cotter keys on the front end. I found  
the passenger side lower ball joint to also have a failed cotter key. There was  
still part of the key in position, but I was unable to remove the residue.  
Turning the nut only turned the ball joint shaft. I had to liberate the nut 
with  the careful use of a Dremel. Another new nut and cotter key and all is back 
 together.
 
Anyhow, what could cause the failure of these two cotter keys?
 
Ned