The Jews were so offended after Stephen said that he saw Jesus "standing" at the right hand of God in Acts 7:55 that they stoned him. Why?

Jesus is pictured as "sitting" at the right hand of God, rather than standing, in most of the New Testament. But Stephen saw Jesus "standing" in Acts 7:55. Then in verse 58 they cast him out of the city and stoned him to death. Did he break a law or were they mad because Stephen was insinuating the fulfillment of a particular prophecy?