For what human reason was Christ crucified? I know that from God the Father's perspective, Christ had to die for the sins of the elect.

However, from a secular perspective, why was Christ crucified?

I know that the Jews were unhappy with Christ claiming to be God -- but secularly, that only makes him insane, not worthy of the death penalty.

There was the time that he over turned a bunch of tables in the temple; but that too seems like a misdemeanor / felony, not death by crucifixion.

Given that he was crucified with "King of the Jews" was the reason that he was guilty of "trying to overthrow Rome" ?
It was the chief priests and pharisees that wanted Jesus arrested. Matthew 26:47 :


  While he was still speaking, Judas, one of the Twelve, arrived. With him was a large crowd armed with swords and clubs, sent from the chief priests and the elders of the people


"that only makes him insane, not worthy of the death penalty" 

Torah, Leveticus 24:16 makes it clear that blasphemy carries a death sentence (although via stoning). More likely, though, he was simply seen as a rebel and a risk, potentially a flash-point for a more visible and popular uprising. That would be plenty of reason for the religious and political leaders to see... economy in a prompt but final action.

I don't know about a secular reason, but: perhaps a political reason that Pilate may have acquiesced to the demands of the Jewish priests; supposedly there is evidence to support some previous hoo-hah involving a row between Pilate and the locals relating to some pagan shields - Pilate refused to remove them, the locals complained to the emperor, the emperor expressed his anger at Pilate's approach. This could lead us to suspect that maybe Pilate didn't want a re-run that could put his position (and likely: life) in jeopardy, so was willing to yield a bit to the locals. Especially if it looked like a riot was likely otherwise.