If DG is a diesel generator that would have to be for the very high initial current draw created by the starting up of the starter motor in order to get it up to cranking speed, along with the additional strain of the compression load of the diesel motor until it starts up.
Take yourself on a bicycle, doesn't it take one heck of a starting effort, compared to the relative ease after you are a meter down the road.
The battery pair just do not have enough capacity to hold
24vdc at that initially severe degree of loading, so the voltage
sags until the initial loading is reduced.