It's not a health issue, they would have said more to allay investor fears. Nor is it a production or performance issue, that doesn't warrant a firing and, besides, he's been a good CEO. He's probably gone and done something totally unacceptable, maybe involving a serious breach of ethics or behaviour. The tone of the announcement suggests true regret in having to let him go - whatever he did he really gave them no choice.
Still, this should blow over when we get the Q4 results and hear the conference call in 3 weeks time. In the meantime we've got a welcome buying opportunity. BCEI now stands out as the cheapest good quality shale play out there.