Sadly, in this day and age, terrorism as the cause comes to mind first.
Time will tell the cause of this.
In the meantime, here's one of the first witness reports:
>>>>Severe storms were hitting the area at the time of the accident. Eyewitnesses reported seeing lightning hit the plane before it burst into flames.<<<<
http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/1123014388624_118423588/?hub=TopStories
QUESTION: will a plane hit by lightning burst into flames?
Eyewitnesses at the scene haven't reported seeing any survivors.
It shouldn't; lightning hits aircraft all the time, and they're designed for it.
However, there are rare positive bolts which are much, much stronger and suspected in some mysterious crashes.