It is quite possibly groundwater.

A mound system leaching field is usually used because ths soil is not suitable for a plain jane leaching field. The soil may be unsuitable because of high groundwater. This may be the case with your soils.

Because the mound is built above ground and the septic tank is built below ground, the sewage must be pumped from the tank up to the mound. Sewage liquid drains into a pump chamber (dosing chamber). A high level sensor in this chamber turns on a pump. When the chamber drains down the pump shuts off.

If you have high groundwater, your septic tank and/or dosing chamber is probably deep enough to be in groundwater. If groundwater seeps into the chamber it will fill then turn on the pump.

The town building dept or sanitarian should have records of your system design and installation. Those records would include groundwater data. Contact them to find out what the groundwater level near your system is. If there is a problem with your system they should be able to provide some guidance.