Economists say that two of the reasons that demand can change have to do with the income of the consumers and their tastes. I would argue that both of these could be causing the change that you describe.
As people get to have more money, they are able to afford more things. Having food delivered to their homes could be something that they were once unable to afford but can now buy with their added income. As people become busier, their tastes might change as well. Being able to save time may become more valuable to them. In that case, they will be willing to spend money to save time by having their food delivered.
I would argue, then, that this change in demand is due to consumers becoming richer and busier and, therefore, more willing to pay to have food delivered to their homes.