It's up to the casting department. Whoever they hire to participate, it is usually the Casting Director (Deena ?)and her Dept. of staff people, who plays a role in picking and choosing the partners and Pro Dancers. As in the case with Maks & Karina, when they returned from their Broadway "Burn The Floor" show, they flew back to L.A. to find out who they were teamed with. Everyone just shows up back to the studios and are informed who the partners are.
It would be more convenient if the Pro Dancer is approached first and asked if he/she would like to go throughthe list of the names and getting the opportunity of choosing for himself/herself, but that is not the case. It's like.... here's the menu and choose want you want from it(if only).