The visor didn't move, it just looks like that:
At first I thought you had a valid point, but upon closer inspection I noticed the grey 'nose guard' (highlighted in red) didn't change place. It's still hanging above his nose, not on it.
One has to keep in mind that in the first shot we're watching from more of a top angle (to his head/helmet) compared to the second shot. This means Luke's eye seems to be behind the top of the visor (like it would if he had the visor on and was being looked at straight).
*sigh*, I'm always struggling to get my images shown....here's a link: http://www.flickr.com/photos/loconut/4931920517/
I hope this explanation isn't too hard to understand, since it's all due to spatial visualization.
Yes, looking up and backwards in a helmet like that would push up the visor and leave it up when he turned around. Makes sense to me.
hmph... okay look...
here is the shot of Luke right after Dak dies.
his visor is fine.
here is Luke looking over his shoulder
his visor is fine (or so it looks to me)
here is Luke looking forward again
his visor is messed up.
Even if it's messed up already when he's looking over his shoulder, I still don't see when it could have messed up.
You're telling me that if you are wearing (sun)glasses while sitting in a chair, when you turn around your shoulder/the chair will push them up on your face such that they don't sit on your nose properly? I doubt it.
I still say it needs to be fixed.