Why Do Readers Flood Bookshops?

If lots of water enters a building in an uncontrolled way, we can say it floods the building. But imagine, instead of water, it's people. So, if lots of people enter a building, we can says they flood it. It means that the bookshops are so full because there are people who want to buy and read Prince Harry's Spare. We often use things like straight from or directly from, just to emphasize that it comes directly from the original source.