Why does orbital optimization remove spin contamination?
Upon further thought, I've summarized it as follows. If anyone comes along with a better explanation, they'll get the cake!

Spin contamination is a fictitious effect arising from not achieving the full CI solution to the Schrödinger equation. For, had we found the exact solution, then the solution we found would be an exact eigenfunction of the ${\hat{S}}{}^2$ operator, as $[\hat H, {\hat{S}}{}^2]=0$. 

In a general sense, orbital optimization can be viewed as simply allowing for more correlation to be recovered on top of whatever other improvements you've applied to your initial Hartree-Fock reference. So, by recovering more correlation, you've brought your solution closer to FCI. In doing so, you've brought your solution closer to a solution that contains no spin contamination. 

However, there seems to be a stronger statement that could be made. This type of correlation seems especially potent in removing spin contamination.