How AP selects its WLC

With this sequence :

[Deterministic] - Primary, Secondary or Tertiary Controller ;

[Initiative]        - attempts to join a WLC configured as a Master Controller ;

[Dynamic]        - attempts to join the WLC with the greatest availability for AP associations ;

It is recommended that you use Deterministic Controller Redundancy over Dynamic Controller Redundancy.

Reference : CLN ( Understanding WLC Technology )
https://learningnetwork.cisco.com/docs/DOC-1375