Complex sentence: It is not clear why, but in 1300 he was interrogated again by the Grand Inquisitor of Parma and found guilty of relapsing into errors formerly abjured and was himself burnt at the stake.
Split into simpler sentences: It is not clear why, but in 1300 he was interrogated again by the Grand Inquisitor of Parma. He was found guilty of relapsing into heresies formerly abjured, and was burnt at the stake.