Having the club head too far underplane during the downswing!
Many golfers for whatever reason poor set up, loss of posture, flat shoulder plane, etc. Cast the club head during the downswing to try and get speed, but unless the various joint extensions are timed well the club head can bottom out early!
I think the best way to stop casting is to find another power source by learning how to properly coil during the backswing ( good sequence to the backswing ) and learn how to ALLOW the club to lag by having the proper sequenced downswing Hip, chest, Arms and Club! Start with short slow easy swings and slowly get longer and faster!