Why would an AP join a different WLC than the one provided via DHCP option 43?

An AP might join a different WLC than specified by DHCP Option 43 if it's "primed." Priming occurs when an AP has previously joined a WLC and stored its IP address in non-volatile memory. This stored address takes precedence over discovery methods like DHCP Option 43. Therefore, if the AP was previously associated with a different WLC, it will attempt to rejoin that WLC first. The other options are less likely to cause this specific behavior. If broadcast or multicast traffic can't reach the WLC, the AP wouldn't be able to discover any WLC, not just a different one. A WLC running a different software version might prevent an AP from joining, but it wouldn't cause the AP to actively seek out a different WLC.