Like SalsaMTB, I'm a (somewhat ambidextrous, right-batting, right golf-swinging, tennis-backhand-switching) lefty who's rightfooted. So I unclip left so I can start off with my dominant leg. I never thought about this when I was learning to use clipless pedals, it just happened that way. I used to ride around a parking lot clipping and unclipping both feet, and ended up being a left-unclipper because it was more comfortable... Since I'm right footed though, it makes sense to me... I also coast right-forward.

The traffic explanation (safer to unclip left so you'll fall out of traffic if you happen to lean the wrong way) never occurred to me but seems like a good idea !