It may be some oddities that you've trimmed out as the others have suggested or it may also be that the wing's leading edge or rib shape is not symetrical from side to side. A noticable difference in the leading edge could easily produce such a snap roll.

I'm often amazed at what some folks consider to be an "adequite" leading edge shaping job. I've seem rectangular and square leading edges that are just barely rounded out and I've seen diamon shaped leading edges that are barely dubbed off with sandpaper and are basically still diamond shaped. Both options will leave you with a wing that doesn't stall nicely compared to a proper shaping job.