Then you either have:
1) a broken pipe at inlet
2) broken tank
3) failed lift pump or backflow preventer on leach field (if you have a sewage lift pump to pump to a higher-level leach field) or after pumping, if your leach field is higher than the tank, he failed to seal the hatches properly after pumping. If this is a new event right after pumping, this or the next one or broken tank from him driving on top of it with the pumper truck would be my first guess.
4) or like above but cracked/displaced septic tank vent pipe it is coming out around/through
5) blocked or slow draining leach field causing backup in the tank
Would have to get a septic/leach field expert in to inspect - usually means pumping the tank dry so you can see where the water is leaking in or out, to check the tank is not backflowing from the leach field, and to inspect the leach field drain pipe manifold and leach field for blockages or crudding up with grease or roots.
