Three things immediately come to mind:
A crack or other imperfection in the glass of that particular bottle.
Uneven distribution of priming sugar prior to bottling.
Possible overcarbonation or infection that is affecting more than one bottle, and that one just happened to be the first to go.
That said, were it my brew, I'd probably crack open a couple/few others to make sure that the issue is limited to just that bottle, as I personally have a pretty healthy respect for potential bottle bombs.