That's an odd thing to say, considering you're not doing the SS setup. You must grab hold of the bar first, so your arms can prevent your shins from kicking the bar forward as they do here. Only then, bend your knees, slowly and carefully, till your shins just touch the bar, but do not move it. That would be one part of the SS setup that you're not doing. Your subsequent reps also do not start with you in the standard pulling position and the bar right over mid foot. You tend to have your hips a little low and the bar a bit forward.
This is the SS Setup, as a reminder: http://startingstrength.com/resource...lls-floor.html
Another part would be step 4, squeeze your chest up. Your back never gets into a good, full extension on this set. Squeeze up. Squeeze harder. Then squeeze some more.
Finally, hold your breath the entire rep. Until the bar is back down on the ground, since the top is not the end of the rep. Breathing out at the top, losing your air there, is losing support when you're still under load.