If you turn the diagram around by 180 degrees and then let Ca <--> Cb, you get exactly the same diagram that you started with. That means the equations have to have the same symmetry under Ca <--> Cb. If both equations have a positive sign you do get the same equations. But if one equation has a negative sign, that negative sign switches to the other equation. So positive works, negative doesn't.
Another way of saying the same thing is, starting out with that symmetric diagram, how would you decide which of dCa/dt or dCb/dt gets the negative sign?