Tim,
I have always heard it was from, just as you say rubbing against, a bag or clothing. I had one that the Wooden base was also worn to match the horn. If the horn was worn everyday for hours I can see how this could happen.
Also, think of Banded horns, the older ones have but one band, the thinking is that it was a sacrificial piece added to extend the life of a horn. As in the band wore out before the horn.
Not a very good Pix but this is my horn that was made in the early 70's you can see some wear on it.
Tim