Correction: you feel sore in your lower back because you never actually extend it.
No problem...this happens sometimes. Watch this video, then try again:
Starting Strength: Video
A few other things to bear in mind:
(1) This is about control, not speed. I'd rather you do a slower, controlled motion to the bottom and get less of a bounce than that you divebomb and lose back extension (once you learn how to do it).
(2) An empty bar set will not tell me much about your form. Form breakdown typically occurs around at least 80% of one's 5RM, and a lot of times form problems with an empty bar set will resolve themselves after weight is added. We'll need to see some weight added to have good advice.
(3) Your knees are way too far back. Get them forward, and break your hips and knees at the same time.