Why did Charles I of Spain sack Rome and imprison Pope Clement VII?

What was his motive? I had understood that he was very zealous with the Catholic religion and enforced burnings of "heretics" (from the Spanish Inquisition which had been begun by his grandparents Ferdinand II and Isabella I). So why, if he was such a devout Catholic, and was a representative in being Holy Roman Emperor, would he attack the the Pope and the Church?