There are multiple fixings throughout the trading day. The Tokyo fixing and the 15:00 GMT fixing are probably the two most active.
A customer who enters an order to be executed at the fix agrees to take the publish rate (by the whatever body set the rate. Sometimes a central bank, sometimes a private company…).
Today there were rumors of a huge buy order at the 15:00 fixing. A bank that had the order would have to buy very aggressively ahead of the time of the fixing or they would lose their shirt.
Why? Today, the rumor was that $4 bln needed to be bought. There is rarely $4 bln of liquidity in the market at any one time…
Lets assume USD/JPY is trading quietly at 82.25 at the fixing. The bank with the order would be obligated to fill the customer order at that price. They would then have to cover their exposure, leaving them with massive exposure
What they do to avoid this is to start buying ahead of the fixing. They fill a great deal of the order and get a sense of how the market is positioned. If there are a lot of sellers, they can buy less aggressively, If there are not a lot of sellers, they would buy as much as they can up until the fix, which is how the market played out today…
