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?
The Son of Man was standing because the throne was already occupied.

In the apocalyptic belief of the time, the Son of Man was God’s divine agent who would one day bring judgment on God’s enemies and inaugurate God’s reign on earth.  The clearest picture of the role of the Son of Man in canonical scripture is in Daniel 7:13-14: 


  “In my vision at night I looked, and there before me was one like a
  son of man, coming with the clouds of heaven. He approached the
  [enthroned] Ancient of Days and was led into his presence. He was given
  authority, glory and sovereign power; all nations and peoples of every
  language worshiped him. His dominion is an everlasting dominion that
  will not pass away, and his kingdom is one that will never be
  destroyed.”


In his speech in Acts 7 describing the failure of the Jewish religious system to understand God’s purpose, Stephen quoted Isaiah 66 wherein God declares, “Heaven is My throne and the earth is My footstool.  What kind of house will you build for me?” (v.49).  God is not contained in temples, Stephen said, but He rules over heaven and earth, recalling the iconic picture of the seated monarch on the throne of heaven with his feet, his authority, on the earth.  

Stephen then added, “Behold, I see the heavens opened up and the Son of Man standing at the right hand of God.”  Stephen was declaring that Daniel’s apocalyptic vision was being fulfilled, that the Ancient of Days, seated on his throne, was even now handing authority over to the Son of Man standing at his right hand.  The everlasting dominion of the Son of Man had begun!  According to the writer of Acts, the Jewish crowd did not embrace this message, and Stephen became the early Christian movement’s first martyr. 

In the following centuries Christians came to interpret the Jewish apocalyptic vision differently.  As their theology of the divinity of Christ developed, Christians came to think of Daniel’s ‘Son of Man’ and the ‘Ancient of Days’ as the same person: the Son of Man was God’s own son, even God himself, and they sat enthroned over the Kingdom of Heaven and Earth as one person.  In Christian doctrine this came to be called the 'Session of Christ', the ‘sitting’ of Christ.  In this light Stephen’s vision was reinterpreted, and various explanations have been offered for why Christ was standing rather than seating.   

But originally, Stephen saw the Son of Man standing beside the throne, and sitting upon it, according to Daniel 7:9-10, was the Ancient of Days.