This was pulled from the configuration page for the camera triggering. Pins 5 and 6 are grouped together and can only be used for either camera triggering or actuator control.
Trigger Hardware Configuration
You can choose which AUX pins to use for triggering using the TRIG_PINS parameter. The default is 56, which means that trigger is enabled on AUX 5 and AUX 6.
With TRIG_PINS set to its default value of 56, you can use the AUX pins 1, 2, 3 and 4 as actuator outputs (for servos/ESCs). Due to the way the hardware timers are handled (1234 are 56 are 2 different groups handled by 2 timers), this is the ONLY combination which allows the simultaneous usage of camera trigger and FMU actuator outputs. DO NOT CHANGE THE DEFAULT VALUE OF TRIG_PINS IF YOU NEED ACTUATOR OUTPUTS.