A random act of sectarian violence happened in Quetta. You want an answer why there's been a sudden sprouting of sectarian murder in Quetta since the US invasion? Here's your answer:
It's not just the Afghan Taliban using Quetta for R & R (rest and recreation), it's also the attraction that poses to sect-ually motivated murderers (I'm sorry organisations) within Pakistan. They come and meet up with their idols, the Afghan Taliban, some locals from the city's Shia community accidentally run afoul of these random wackos, and the next thing you know, bad attitude mixes with sectarian prejudice and the situation explodes into an incident like this.