Why did Robert Xie face four trials for Lin family murders?

Robert Xie has been found guilty of murdering his brother-in-law Min "Norman" Lin, 45, Mr Lin's wife Yun Li "Lily" Lin, 43, her sister Yun Bin "Irene" Lin, 39, and the Lins' two sons Henry, 12, and Terry, 9 on July 18, 2009.