Strange...

[MuskieBait]

Those two explanations are likely. Did you see any pigtail at the end of the line (indicating a knot failure) or was it a fairly clean break (simple line break)?

If you are fishing rocky bottom, your line could suffer nicks a few feet above the hook/lure. Did you check the line for any damage before tying a new knot? There could already be a weak point in the line with the new knot.

When you check a knot, it may seemed ok...but 15lb is fairly strong and sometimes a bad knot wouldn't snap when you check the knot.