Exact same thing happens to me, so I had a DPT watch me bench. You're not going to believe what my problem was: tightness in either my left pec, right pec, or both.
Essentially, she saw two things going on. As I lowered the bar to my chest, my tight left pec resisted going down all the way so the right side of the bar descended a bit more quickly leading to more weight on the right side. My tight right pec (or shoulder) dipped towards my feet to take pressure off the shoulder and my QL gripped to try and stabilize my entire body.
So, at the bottom of the movement, I looked like the letter "C" if looked at from above. Her suggestion was to keep stretching my pecs and work on shoulder mobility.
Hope this helps.