Echoing the answer provided by /u/brojangles, most scholars agree that the most likely charge was that he was being hailed as "King of the Jews." Interestingly, in his most recent book, How Jesus Became God: The Exaltation of a Jewish Preacher from Galilee, New Testament Scholar Bart Ehrman sheds a little more detailed light on his (and many other) scholarly opinions on why Jesus was crucified. Here is a great quote from his book:

...when the authorities arrested Jesus and handed him over to Pontius Pilate, the consistent report is that the charge leveled against him at his trial was that he called himself the king of the Jews. If Jesus never preached in public that he was the future king, but this was the charge that was leveled against him at his trial, how did outsiders come to know of it? The simplest answer is that this is what Judas betrayed.

Judas was one of the insiders to whom Jesus disclosed his vision of the future. Judas and the eleven others would all be rulers in the future kingdom. And Jesus would be the king. For some reason— we’ll never know why— Judas became a turncoat and betrayed both the cause and his master. He told the Jewish authorities what Jesus was actually teaching in private, and it was all they needed. They had him arrested and turned him over to the governor. Here was someone who was declaring himself to be king. [Page 121-122]

Of course being King did no refer to being king on this earthly realm. As most Christian scholars believe, Jesus was an Apocalyptic Rabbi (like John the Baptist and many other of their time) who preached that the end of the world was near, and it was of this next world that Jesus most likely preached that he would be king.

It's also worth noting is that Pilate had a history of dealing with sedition in very harsh measures. In one of Josephus' writings (Antiquities 18.3.2), he writes about how Pilate had no problems putting an end to insurrectionists with violence and hostility. He was known as a governor whom had little respect for Jewish customs and religion and did not value the lives of the people whom he ruled over.