Why did a crowd gather when the friends were ready to go on their holiday trip

I think it is because everybody was looking for their train,

We learnt, afterwards, that the train we had come by was really the Exeter mail, and that they had spent hours at Waterloo, looking for it, and nobody knew what had become of it.

Because when they come out of their house they carry too much luggage that everybody thought  that they are going to attend wedding and Harris was groom.