Complex sentence: There appear to be several contributing factors to the abandonment of Mayapan, around A.D.1420 a riot was started by the Xius against the Cocom that ended in the death of nearly all (if not in fact all) of the Cocom lineage.
Split into simpler sentences: There appear to be several contributing factors to the abandonment of Mayapan. Around A.D.1420 a riot was started by the Xius against the Cocom which culminated in the death of nearly all (if not in fact all) of the Cocom lineage.