I went through a bad leader popping episode last season on a trophy trout lake here in NZ. I just kept breaking off as soon as I tightened up on a fish. They were all big, so each break off was significant. Even when I went up to 10lb test and more, I just kept popping off..
When I got home I tested it out with a pal. Although it as brand new, I blamed the tippet material, which was supposed to be the best, Three turn surgeon's, well wetted, popped almost every time. The we tried four turns and tightening the knots better. No more breaks. Went back to three turns, wet and tightened fully, no more breaks. Haven't had one since.
In a fishing situation, I was just tying the knots too quickly and not really reefing them tight.
It appears that the tiny amount of slippage creates a burst of heat that weakens the knot to a critical degree, and it breaks. So make sure you wet those knots and really pull them tight. Making sure they have no overlapping turns is important too, so slow down and make the knot count..