Not sure it seems to be pretty random. I know we have tried sitting tanks at different ranges and sorting them in raid groups. It just seems random. If you don't want a specific tank to get the aggro you can maybe try to guard him and see if that helps though guarding a tank is usually a bad idea.
A guild on our server does this exact thing to influence this. They have everyone stand behind walker with one tank, and that tank guards the tank that stands out and picks up the warriors. All of the trenchcutters run to the other tank.
They place the guard on the OT as they're killing the last droid.