[spritzer]

On the side, do you mean the phones are resting on one of the earpieces?  That puts strain on the arc when you should ideally store them sitting on the forks since they are reinforced. 
 
Since you are storing them inside a cabinet it shouldn't be a factor but direct sunlight and electrostatics don't mix!! 

 

[tksc14724]

Sorry poor choice of words. On their back would have been better. Nothing's under stress at all.

 

[spritzer]

Hmm then it is rather odd.  Is it the fork that breaks (i.e. the piece which holds up the earpiece) or the plastic arc (which spans the head) that breaks? 

 

[tksc14724]

The fork broke once but that was a very early sample. I believe it was from when Stax had just moved back into the UK and there were rumours of problems within the company. Every other time it's been the arc. Always on the left (including the fork) and always in exactly the same spot i.e. immediately above the vertical runner for speaker height adjustment.