Strange...

[yakfish]

I'd say your first explanation is the right answer. I have had lure spoons in particular that are bad about wearing through line at the know. At the very least I would put a split ring on there and tie to that or a snap and swivel. Spoons have an edge that may not appear to be very sharp but with the wobble of the spoon and the constant tension on the line it can act as a mini saw and cut through the line. I don't fish with spoons tied directly to my line anymore for this exact reason.