The slide is hitting the chamber's block....There's quite a few threads with pics in them....I think it boils down to a timing and tolerance issue, the slide is coming back too fast and the Barrel Block isn't getting down out of it's way fast enough. Over 150-500 rounds a little deformation starts forming, and gets worse as you shoot it. Only heard of one or two cases where the SR9 locked up because it got so bad.
Generally one or two trips to Ruger can fix it, but each gun seems to be a bit different.
Mine has a very very small deformation which is what I'd say is normal wear, others are worse....the majority though, isn't showing major signs of peening.