Squat Form Check 225 lbs Low Bar

Heres the video from2 weeks ago:

http://www.youtube.com/watch?v=UNXepMFsr84

Heres what I did today. I did 240 last workout, but I thought I was going to plateau so I decided to drop it down today.

http://www.youtube.com/watch?v=Zt7YAOJYHDA

When I go below parallel I can't seem to keep my back from rounding at the bottom.

edit** I'm also on my 4th set, so a bit of struggling.