[Kevin Sevcik]

I'm guessing poor sprocket alignment? If the sprockets aren't in line with each other, you could get significant wear from the side plates of the chain rubbing against the side of the sprocket. That's about the only way I can think of that you'd get that kind of wear pattern on the sprocket. I assume that once the sprocket was worn thin enough the teeth sheared off and then the sprocket was replaced.