Ya, I had this happen last weekend. Leader/rod mainline got twisted around the downrigger line a few times, eventually the friction wore through the downrigger line and pop - 100$ to the bottom.
Two causes I figure.
1) I was letting the gear down too fast and a) the dummy flashers were whipping around the tuna cord which lead to leader tangles etc.
2) the spoon was getting wrapped around the down rigger line.
But also check for this one which I think was the main issue I was having:
3) I wasn't paying attention to the clip, I run a long clip line so I avoid leaning far out over the boat etc, and there have been times where the clip line flips back around downrigger line a couple times when things are slack when I'm re-clipping in the line I'm fishing. Usually it fixes itself when there's tension right before I send it down, but at least a few times this month it hasnt, and it causes a real mess of tangled line around the downrigger line. I think this was the culprit when the whole shebang broke off.
I fish braid on the rigger and braid on the reel with a 30-50ft top shot. Braid on braid isnt good and the thin line will cut the thicker line.
Main thing is dont rush, pay attention to the details.
"Fast is slow, slow is smooth, smooth is fast"