Whiz, I think it is very unlikely that the string broke because it was under too much strain. I think it takes something like 4 strands to be safe and the rest just make it thicker so the
nocks
fit properly.
If the strings had been cut through in the serving process, the strings that were still in use would have been under tension and the others would be loose-ish. The loose ones often show as a spiral twist along the string.Did the string look nice and round when it was being used or did it have that liquorice twist look?( probably too late to ask that now)
I wonder if it could have anything to do with a platform tab; some platforms may have a sharp edge that could contact the serving about 1" above the nocking point. That may be a very unlikely cause; but it is worth checking out. If it is the reason for the cut strands, it will probably happen on the next string, too. Not worth the risk!