Automatic Bus Transfer Switch (ABT)
It could be that your rig has an Automatic Bus Transfer (ABT) switch, or equivalent circuitry built in. These often provide two functions. They are normally generator-seeking, meaning if a generator is hooked up the electric loads will switch to it even if shore power is still available. Now they often include two circuitrys, 1) a time delay of approximately 45 seconds from the time a generator is hooked up and powered, until it then switches over to the generator. This gives the generator a slight warm up before being "hit" with the electric loads. 2) Some also include generator stabilization sensing, such that they don't switch until they sense the generator is stable then they switch to the generator.