From what I see M7/M12 are just providing level shift. M6 and M5 are most probably same or ratioed. If Vds of M6 (defined by one Vgs above gnd) is bigger than Vds of M5 (defined by the output CM voltage) then the drain current of M6 is bigger than that of M5 (due to finite gds) and is mirrored into M9. The output CMFB through M3/4 will have to increase the Vgs of M5/M10 to compensate which will increase the current through them but also increase the current in M6/M11. It is kind of chasing its tail until some balance is reached perhaps at a higher current.