this could have several Problems. To probably help you, i have to know the manufacturer and detailes type of your generator.
For example it is possible that your generator has an automaticstart, which starts the generator if your battery voltage goes under "low-level". This could happen when the charger for your batteries that charges them with the shore power is not switched on or has an failure. But that's only on possible reason...
If you give me more details i can help more in detail.
PS: Could you write in German, because you are from switzerland?