When the solenoid operates it cycles on/off/on/off quickly, (I.e. over a period of a second or less.)
The current being drawn by the 2nd battery causes a voltage drop on the cable between the main battery and the isolator. This voltage drop causes the isolator "see" less than 12.7V and it switches off. When it switches off there is no longer a current draw so there no longer the voltage drop so the solenoid sees more than 13.5V and switches on again... this cycle is repeated possibly many times.
The best solution is to reposition the SBI closer to the main battery. If this is not possible, the effect can be reduced by using thicker cable between the main battery and the 2nd battery.
The 2nd battery has been heavily discharged (E.g. running a fridge overnight) and the owner starts the motor and leaves it idling to recharge the battery, but then finds the SBI12 cycling on and off slowly (E.g over a period of 10, 20 or 30 seconds Etc.)
At idle, the alternator cannot produce enough current to charge the 2nd battery.
Run motor at higher RPM to allow alternator to function effectively.
The 2nd battery has a battery charger connected which charges to a high voltage, E.g. 16V. SBI cycles on/off even with motor not running.
When SBI senses more than 15.5V it disconnects, allowing start battery to discharge (while aux battery still being charged at 16V). When start battery falls below 15V, SBI re-connects and cycle repeats.
Wait until batteries fall below 12.5V and SBI switches off before connecting this type of higher voltage battery charger. It is possible that a combination of both the above scenarios may be encountered.