If you look at a variety of left-handed players- Melzer, Nieminen, Bellucci, Klizan, Lopez, etc. you will see that there forehands all have a large amount of spin on them. Even S&V players like Llodra and Muller. Is this just a coincidence? We'll see.
It has been scientifically proven that left-handers are more adept at using their right hand, than right-handers are at using their left hand. I don't know why this is, it just it.
So I tried an experiment. I (right-hander) played tennis with my friend using my left hand. I've been practicing for a while the other hand, so I'm pretty good at playing left-handed. What I found, is that it is very hard to get rid of the spin on my lefty forehand. With less coordination, you hit looser, which means generally there is more topspin on the ball. On my less coordinated hand, I naturally hit with more spin.
Rafael Nadal was in a similar situation. He is naturally a right-hander, believe it or not. Uncle Toni made him switch to left. Why? Probably because it would be more natural for him to get more spin on the ball lefty. When you have more control of your hand, your wrist is automatically more rigid, like it or not. Dull switched to left hand, allowing him to hit with loose wrist and generate insane amounts of topspin. And we all know Dull's game is based around topspin solely.
Now, left-handers have slightly less control over their dominant hand than right-handers do of their right hand. (In exchange of greater control of the other hand of course). Maybe something on the left side of the brain is responsible for these changes. Maybe not. Whatever. But it is possible that is natural for these lefties (Melzer, Nieminen, Falla, Bellucci, Klizan, Lopez, even Melzer's younger brother) to hit with a looser wrist, thus allowing more spin.