Sudden massive increase on house offer?

Good morning people, my partner and I put an offer on a house about 10 days ago for 492k, which happens to be with the same estate agent we are selling with, they informed us that this was the top offer and the vendor is happy to wait and see what happens with our property before they accept any other offers.

Yesterday I received a call to say we have an offer on the property we are selling... good news, however bad news someone has offered 520k on the house we offered on which has a guide price of 485k. Does this seem a bit suspicious to anyone else? Do people come and offer 35k over the guide price just out of nowhere?