Once a house in the street goes for a huge sum, because a buyer got talked into it, every seller will be pushing for the same result.
It's ridiculous, but for some people an extra 100K is small change, if they want a house they want it, and then that becomes the price guide for the area - guide being the operative word, ultimately the market will set the price but a lot of people have money to burn and will be talked into overpaying by a skilful agent.