It happens, it is very rare but it happens. Something like that happened to me once. 90 percent of the time it happens because of manufacture defect, but can also be caused by leaving racquets in really cold conditions (cold causes the racquet to crack). Of course you can also cause the racquet head to fly off by smashing the racquet and causing a crack, then a short time later the racquet blows up.
But as said very rare for that to happen, especially in the pro ranks.