Initial thought is that these controllers are on the same subnet. And the one taking all the AP's is the master.
So I found the following for you - ref : Cisco Wireless LAN Controller Configuration Guide, Release 7.2 - Chapter 1 - Overview [Cisco Wireless LAN Controller Sof…
Cisco Wireless LAN Controllers
When you are adding lightweight access points to a multiple-controller deployment network, it is convenient to have all lightweight access points associate with one master controller on the same subnet. That way, the you do not have to log into multiple controllers to find out which controller newly-added lightweight access points associated with.
One controller in each subnet can be assigned as the master controller while adding lightweight access points. As long as a master controller is active on the same subnet, all new access points without a primary, secondary, and tertiary controller assigned automatically attempt to associate with the master controller. This process is described in the "Cisco Wireless LAN Controller Failover Protection" section.
You can monitor the master controller using the WCS Web User Interface and watch as access points associate with the master controller. You can then verify the access point configuration and assign a primary, secondary, and tertiary controller to the access point, and reboot the access point so it reassociates with its primary, secondary, or tertiary controller.