Check the above post's advice first.
Here is the advice from Saniflow website of what to check if the motor activates after a flush.
Repeated starting of pump
Sometimes the pump will start up more than once after a flush. Please check the following:
Overflow Tube. Lift the lid and check the overflow tube (typically a black hose running from the fill valve to the flush valve) to see if it is attached to the overflow tube in such a manner that water sprays into the overflow tube. If so, move the clip so that water now sprays into the reservoir. This type of toilet does not need to have the trapway refilled after a flush. Or, some silicone squirted into the tube will stop water from flowing through the tube.
Flush Valve. Also check for leaking of the flush valve into the toilet. To easily check add a few drops of food dye into the tank until the water is slightly colored and then observe the back wall of the bowl for any colored water to drip down. If this happens it means that the flush valve is leaking and need to be replaced.
Fill Valve. Observe the water level in the tank. It must be level with the water level mark. If the water level is above the watermark, the float of the fill valve needs to be adjusted down. Similarly, if the water level in the tank is too low, the float must be adjusted up.