Theory #11, Limited Space
I admit this really isn't a theory. Perhaps a better description would be a qualifier. The underlying assumption to all of these posts is that driving in the left lane can be irritating to other drivers if you are doing so outside of the unspoken rules. But those same rules say that there are times when driving in the left lane isn't all that bad.
Any road will at times get so loaded with traffic that no matter what lane you are driving in you are going no where. There's just so much space. In crowded conditions no one really cares who is driving in the left lane because they are just frustrated they aren't moving. I guess you could say that there is no distinct target of frustration in a traffic jam except for the traffic as a whole.
So traffic may cause people to drive in the left lane, it happens.