An error occurred in Webkit (Safari) that created a condition the system could not handle. For a more complete explanation you would need to talk to one of Apple's Webkit developers (a solid grounding in software engineering would be of great benefit in that conversation
)
There is no indication of what triggered the incident and whether it was hardware or software in origin.
If this happens again you might want to run some diagnostics. (I recently had a spate of unexpected shutdowns, occasional inexplicable errors,
etc that turned out to be the result of a swollen battery. — three cheers for appleCare!)
