Just a good ol' fashioned case of wobbly-patella-bad-kneepcap-tracking-itus. There is a proper medical name that I always screw up/invert. It's something along the lines of femoral patella-something-or-other. In my case it was actually caused by my bad feet. Exercise and stretching helped a lot but orthotics to suppport my collapsing arches essentially cured it.