Burnt out another MH, why?

[mark]

Running two 250W 14k, DE Hamilton's in PFO mini-pendants powered by AQ2-250HQI-120V PFO dual ballast, 8 hours/day. Pendants are mounted in a enclosed hood vented by a 5 inch muffin fan ducted up from basement.

One bulb has been going for a year (happy anniversary to me, tank up a year on the 17th) but only got 6 months from other and now it's repacement's burnt out. Though not much of a trend, that's 2 bulbs that have only lasted 6 months. Should I be looking for a problem with the ballast or just bad luck?