Because it insinuated that He was God. When Moses asked God what His name was, He responded with "I am". By saying He came before Abraham, and identified Himself as "I am", to the pharisees it basically mean this guy was calling himself God. And back then that would get you killed on the spot.
Additionally, only the Patriarchs (Abraham, Isaac and Jacob) actually interacted with God, oh and Moses. To them God never actually showed up anywhere, He just sent prophets. So a random guy claiming to be above the prophepts and God himself is crazy talk.
Why did that Infuriate the Pharisees so much? I think it says they picked up stones and Jesus proceeded to tell them they were of their father Satan.
Additionally, only the Patriarchs (Abraham, Isaac and Jacob) actually interacted with God, oh and Moses. To them God never actually showed up anywhere, He just sent prophets. So a random guy claiming to be above the prophepts and God himself is crazy talk.