Hey Mattias
Looks like you're upper back/thoracic spine isn't really staying in tight extension. As a result the bar is moving around and you're probably trying to keep it stable with your arms unconsciously. Lift your chest, and pull the elbows down when you take the breath. Maintain the upper back extension as you lean over and keep the elbows from cranking upward.
Also, find a bar with a center knurl or put a piece of athletic tape around the middle so it's less likely to slide around on your back.
It's a little thing to help keep the bar in place, but I'm surprised how often I see it.