Both.
Fatigued brass certainly doesn't help and could be a contributing factor but its never been a problem in my CZ's, 1911's or Glocks.
I've had at least 30 cases bulge and 1 blow out out of approx 450 rounds through the x5. I've probably shot 150,000 reloads through other 9mm guns over the last 6 or 7 years and NEVER had a blowout of any kind.
I could live with the bulging but having a blow out and an extractor leaving my gun mid stage would ruin my match.