The Prologue starts the play by emphasizing several key themes of this play, namely feeling pity for those who have fallen, no matter what their past, and the revelation of truth. Explaining that this play concerns the rise and fall of important people close to the king, the Prologue sets the tone of the play. It is not comedy, he says, but more a political thriller. The emphasis on pity indicates that none who will fall are really evil but that they were perhaps misguided or unlucky, and they don't deserve for us to think badly of them.
Freshly back from demonstrations of wealth and power in France, Buckingham is barely able to contain his rage at Wolsey, who he believes is a sinister figure who is attempting to commandeer the power of the king for his own ends. Norfolk's urging barely calms Buckminster, and Buckminster openly accuses Wolsey of treason. Wolsey certainly has his own bad opinions about Buckingham, briefly sounding his plan to get Buckingham's (former) estate manager to testify against him. Buckingham may accuse Wolsey of treason, but Wolsey has the power and means to prove that Buckingham is guilty of that same crime.
Buckingham is the first character we meet, so we tend to believe his accusations, though he gives no clear explanation of what he thinks Wolsey has done wrong. Being arrested helps his case, as it proves that Wolsey was plotting against him offstage. Though the audience has no real proof whether Wolsey is treasonous or Buckingham is to blame, we believe Buckingham as our first witness to the evil of Wolsey. Buckingham is also the first to fall.