What are you casting? A float? A leger weight? A feeder? :confused:
An over filled spool will cause most of the problem as will casting a heavy weight and allowing the line to continue to peel off the reel after the float or feeder has landed.
A little more info please and a more accurate assessment might be forthcoming 😉
Are you casting with a fixed spool or centrepin reel? I trust you are not trying to cast with the bale arm closed and using a slack clutch as this would cause over-runs.
As feeder has suggested you may have completely the wrong line loaded on your reel.