Fatigue not so much from constant repetitive forces over a number of years as one good hard landing recently. Going over a drain cover or divet on the ramp could be final stressor.
I'm not aware of any material change to the landing gear construction on the 320 compared to years gone by. My response was to the comment about sources already saying it's a manufacturing defect. I'd be surprised, personally.
I've heard of one outfit that had over half its landing gear wheel units over torqued at the hangar and some units (after re-inspection due to cracking) under torqued.