I think we're all going to assume that your dominant hand is the reason why you choose to space with your left thumb or your right thumb, but it's possible that there are cultural factors in play too. In hockey, it's more common for American players to shoot right handed, but in most other areas of the world, people have a bias toward shooting left handed.
I've noticed that one person here mentioned that he's a lefty thumber when gaming and a right thumber otherwise. It might be that people who have grown up on FPS games that use WASD controls and a mouse are more likely to space with the left thumb since they get so used to it from gaming. Those of us who grew up with Atari Sticks and Zork wouldn't have had these experiences early on.
I'm right handed and my habits are pretty consistent with right handed people in most areas, except one. I'm a southpaw when doing martial arts. I'm not sure how I ended up that way, but it could be that the first martial arts class I took was three months of Wah-Lum Kung Fu (Praying Mantis style) which does a lot of parrying with the weak hand and a counter strike with the strong hand. I went on to other martial arts, but it's possible that the three month intro I had set me up to be a southpaw.