From CNN:

"He came over the top of a stingray that was buried in the sand, and the barb came up and hit him in the chest," said Stainton.

Wildlife documentary maker Ben Cropp, citing a colleague who saw footage of the attack, told TIME that Irwin had accidentally boxed the animal in. "It stopped and twisted and threw up its tail with the spike, and it caught him in the chest," said Cropp. "It's a defensive thing. It's like being stabbed with a dirty dagger."


Looks like he didn't even know that it was there. Erwin got on top of it and the ray thought that it was being attacked, and tried to defend him self from Erwin. Had it only known. Sad loss, I have always liked watching him.