Is it an overhand knot? Happens a lot using shot. I sometimes use a 2 hook rig with a 1/32 jig on the bottom and an Aberdeen above with the shot below the top hook.A few things can help but none of them are foolproof. Instead of letting it free fall straight down gently swing it out away from the boat and let it fall at an angle. If you do cast just lob it out from the side with no english from the rod tip. Make sure the shot is lighter than the jig so it doesn't beat the jig down. Clam style shot are easier to get the knot off of. Check it frequently before it gets really tightened.