Our volvo 240 bucket curls by itself then crams hydraulic pump even if the Safety arm is up. I can stop it by moving the lever out of center I may hold for a bit then take off by itself.
Lay the arm and boom out, pull a vacuum on the tank. Remove the bucket spool. It sounds like your centering spring has come loose. Be prepared for a rush of oil from the bucket circuit equalizing. Be careful and good luck.
Good day Information may be of help. Kind regards Uffex