In the beginning of episode 1 of the anime, Aqua makes fun of Kazuma for how he died. She then says that she has, "Well... I've vented enough stress for now," before continuing her explanation of the choices that Kazuma has in the afterlife.
Is this a joke or did Kazuma really die from shock of possibly being run over by a truck?
I hope he died of shock.
Kazuma died because of heart attack.
Actually, the girl will survive even if Kazuma didn't save her. The vehicle that was moving towards the girl was just a slow tractor. After he pushed the girl, Kazuma died because of heart attack, he thought it was a truck and he was scared to death.
My personal opinion, and this isn't proven anywhere but: He did in fact see it correctly and attempted to save the girl but both of them died in the accident, and she didn't want to tell Kazuma that his life ended in tragic failure and in vain. Thus the "getting out some stress" comment. Now while that is debunked by how bad a liar she is, at this point she was still a Goddess and had time to think it up, and her personality does change a bit after joining Kazuma in the alternate world.
In episode one of Konosuba, it was shown that he saw a "car" about to run over a girl so he pushed the girl out of the way. However, after he died, Aqua showed him that the "car" was actually a tractor that has already stopped and was never going to hit the girl.
Kazuma then asked how he died, then Aqua told him how he died of shock and wet himself, she also told him how every doctor and his family was laughing at how ridiculous his death was.
