What happened? tube blew the rim?
This kind of failure is typically caused by excessive wear on the brake surface of the rim.  

Every time you apply your brakes, you are polishing small amounts of metal away from the rim. Eventually, the rim gets too thin and weak, and will crack, like you have seen in your photo, from the normal inflation pressure of the tire. 

In general, this means tht the rim has been used beyond its life. If it is a new, or lightly used rim, it may be a manufacturing defect.