I was standing behind my car yesterday and couldn't help but notice that I've got a significant amount of negative camber in the rear of my s5. Matter of fact, it seems to be worse on the driver's side than the passenger--does that sound right?

First of all, what causes this? I've seen Rexes that have this issue, but some are worse than others. What causes this to get worse over time? Is it an indication that I need new springs/shocks? If I were to upgrade my suspension and get new shocks/springs, will the negative camber be corrected? Why is the negative camber worse on the driver's side (no one was sitting in the car, and it had a full tank of gas)?

Thanx

 

If the car is lowered a lot then the rear camber becomes excessive. You have to buy an adjustable camber link to fix it.
If your springs are stock, they've either gone soft so that the car actually is lowered, or something is screwed up. It may be possible the bushing are extremely worn out, or something is bent. Which would also explain why it's different left to right (bent).
At stock ride height camber should be fine.
Good luck