Many myths surround why he was arrested. The gospel account claims it is because he claimed to be the Messiah. However, it is questionable whether this was against Jewish law (talmud). And even if it were, why would Pilate concern himself with Jewish law?
One suggestion is that Palm Sunday was not as is often spun in the gospel accounts to match some prophecy of the messiah, but rather may have been "street theater" to mock the Imperial Roman procession that would have been happening at the same time with Pilate at the other side of the city. It may have been a kind of demonstration against Roman rule, getting him in trouble with the Roman authorities and law.
Judi Magness, University of North Carolina