Could be an erroneous reading due to a bad sensor.
Could be some high energy demand device is on.
Could be a short circuit somewhere.
Could be a bad transfer switch.
My first place to look in our coach would be to see if the inverter is set to too low an amperage allowance from shore/generator power. When I use my 3000w portable generator, I set my inverter to "power sharing" at 10 to 25 amps so the generator's breaker will not get kicked out. If the sum of electrical devices' demand (water heater, induction cooktop, convection/microwave, etc.) tries to overtax the generator, then the house batteries will power share with the generator and show a discharging condition. When those devices are later turned off, then the inverter will use generator power to recharge the house batteries.