CREATE TABLE table_26518 ("Co-king" TEXT, "Relationship to Monarch" TEXT, "Crowned" TEXT, "Co-kingship ceased" TEXT, "Reason" TEXT, "Monarch" TEXT)

Name the reason for 20 july 1031
SELECT "Reason" FROM table_26518 WHERE "Co-kingship ceased" = '20 July 1031'
SELECT "Reason" FROM "table_26518" WHERE "Co-kingship ceased" = '20 July 1031'