During an exercise, SM-2 standard missile exploded on launch, damaging the Aegis Class destroyer, The Sullivans:

A US Navy guided missile destroyer was damaged by a surface-to-air missile that exploded shortly after launch during an exercise off the U.S. Atlantic coast on Saturday USNI Reports. 

 The US Navy confirmed that a Raytheon made Standard Missile-2 (SM-2) test missile exploded after suffering a malfunction as it was fired from the guided-missile destroyer USS The Sullivans (DDG 68) during a planned missile exercise off the coast of Virginia. There were no injuries and only minor damage to the port side of the ship resulting from missile debris. The ship returned to Naval Station Norfolk for assessment.

Thankfully, there were no injuries.

BTW, the explosion appears to be of the missile booster, not the warhead.

In descending order, the likely causes of this are:

• Cracks in the propellant, causing an uncontrolled burn.
• A flaw in the engine casing.
• A flaw in the nozzle.
• Something else (Yeah, I know it’s kind of a catch all)

What the navy is probably doing now, in addition to collecting bits of the missile and figuring out what happened, is pulling all the missiles from that production lot for inspection, and they will probably increase inspections more generally.

My (slightly) informed guess (I used to work on missiles at Lockheed-Martin Missiles and Fire Control) is that this was an older missile, and that cracks developed over time as a part of the aging process, but opinions are like assholes, everyone has one.