What caused Hide's stomach wound? Like the title says, what caused Hide's stomach wound?
I'm assuming you are talking about the anime version, which is different from the manga version.

Although it was not shown on screen, it is highly suggested that Hideyoshi was caught in the fight between Seidou Takizawa and the ghoul Noro. In fact, we last time we see Hide before he meets Kaneki, he had been noticed by Noro right after he ate Takizawa's arm.