Jesus Is Mocked (Matthew 27:27-31)
Then the soldiers of the Prefect took Jesus into the Praetorium [Prefect's Palace], and they gathered the whole Cohort [600 men] before him. And they stripped him and put a scarlet robe on him, and twisting together a crown of thorns, they put it on his head and put a reed in his right hand. And kneeling before him, they mocked him, saying, "Hail, King of the Jews!" And they spit on him and took the reed and struck him on the head. And when they had mocked him, they stripped him of the robe and put his own clothes on him and led him away to crucify him.
Why did the Roman Centurions do this? To understand, you must recall the Star Prophecy:
The "Star Prophecy" was a Messianic reading applied by radical Jews ... to a text from the Book of Numbers 24:17:
There shall come a star out of Jacob, and a sceptre shall rise out of Israel, and shall smite the corners of Moab, and destroy all the children of Sheth.
which was often employed during the troubled years that led up to the Jewish Revolt, the destruction of the Second Temple in Jerusalem (70 CE) and the suicidal last stand of the Essenes at Masada in 73 CE. The Star Prophecy appears in the Qumran texts called the Dead Sea scrolls. This was the prophecy that was of such importance to all resistance groups in this period, including those responsible for the documents at Qumran and the revolutionaries who triggered the war against Rome.
http://en.wikipedia.org/wiki/Star_Prophecy
The Radical, militant extremists among the Jews — the Sicarii assassins and the Zealots — believed that the coming Messiah would be a "warrior Messiah" who, like Joshua and David, would slaughter and subjugate the heathens. These Jews, a strong and vocal minority, were so convinced that YHWH-God meant for them to dominate the world that they drove (by force) the Jewish nation to war against the Roman Empire.
Indeed, this notion of world domination was so prevalent amongst these militant extremist Jews that the Romans were well aware of their intentions. For example, numerous classical writers from the 1st century knew about their world domination aspirations:
But now, what did the most to elevate them in undertaking this war, was an ambiguous oracle that was also found in their sacred writings, how, about that time, one from their country should become ruler of the [whole] habitable earth.
Josephus, Jewish War, 6.5.4 (75 AD)
Prodigies had occurred, which this nation, prone to superstition, but hating all religious rites, did not deem it lawful to expiate by offering and sacrifice. There had been seen hosts joining battle in the skies, the fiery gleam of arms, the temple illuminated by a sudden radiance from the clouds. The doors of the inner shrine were suddenly thrown open, and a voice of more than mortal tone was heard to cry that the Gods were departing. At the same instant there was a mighty stir as of departure. Some few put a fearful meaning on these events, but in most there was a firm persuasion, that in the ancient records of their priests was contained a prediction of how at this very time the East was to grow powerful, and rulers, coming from Judaea, were to acquire universal empire... the common people, with the usual blindness of ambition, had interpreted these mighty destinies of themselves, and could not be brought even by disasters to believe the truth.
Tacitus, Histories, 5.13 (100 AD)
There had spread over all the Orient [East] an old and established belief, that it was fated at that time for men coming from Judaea to rule the world. This prediction... the people of Judaea took to themselves; accordingly they revolted and after killing their governor, they routed the consular ruler of Syria as well, when he came to the rescue, and took one of his eagles.
Suetonius, Life of Vespasian, 4.5 (120 AD)
Please also recall that the Sicarii assassins frequently murdered Romans and alleged Roman collaborators, for decades in the lead up to the Jewish Revolt (66-70 AD).
Thus, the Romans had legitimate reason to fear these Sicarri assassins, their Zealot allies, and the "Messianic" fervor that animated them.
This is why the Roman Centurions dressed the Messiah up in royal purple robes — the clothing of the Roman Caesars, the Emperors.
The Romans believed, and feared, that Jesus was a Zealot - Sicarii - Assassin - "Warrior Messiah". In direct response to their understanding of the Jewish "Star Prophecy", the Romans briefly bowed down to the Messiah to mockingly say, "you will conquer Rome, you will slaughter and subjugate the Roman Empire, you will do what no enemy has done in four hundred years, you will break us and force us to worship you and make us your slaves?!?"
NOTE: It is interesting to note that Pilate was sent to Judea in 29 AD — exactly when John the Baptist began his pre-Messianic ministry (Luke 3:1). Author Glenn Kimball asserts that Pilate was sent into Judea by the Roman Emperor Tiberius specifically to investigate this John-Jesus Messianic movement, and to arrest and eliminate the Messianic claimant. This would have been specifically to thwart what the Romans legitimately saw as a grave threat to their national security — to wit, these brutal "Messianic" dreams of the extremist Sicarii assassins and their Zealot allies for world conquest and domination through slaughter and bloodshed.