Could be the knot to the leadcore has failed because the mono was damaged when tightening, a frap up or damaged line due to excessive twist or wear. I have seen loads of dodgy knots to leadcore or swivel if fishing straight through.
The rod action is also a factor...
I used to fish large solid bags using 15lb big game and beefy 3.5lb rods but they were more through action, so the acceleration, loading and release on the cast were 'smoother' than on a faster actioned rod.

In reply to Post #1
Just a thought but could it be a cracked ring be causing the problem? Sychro in 15lb should be pretty bullet proof as already stated. Rub the line hard on each part of every eye and inspect it after. A hairline crack would be near impossible to see but that should reveal if there is one present.

In reply to Post #1
The strain on the line is too much as others have said, you need a leader of some sort to take the strain of the cast.

Tapered leader or tapered line will solve it. Then you can whack it as hard as you like.

In reply to Post #33
if allowed in my experience casting 4 oz or there abouts long range a 0.32 mainline and a tapered leader has never let me down hitting 120 yards, tbh lots of people say they casting solid bags 100 yards plus and there no where near,its not an easy thing to do