That's a process called maceration, which happens when most fruits are mixed with sugar. Basically, the grains of sugar bust the cell walls. This softens the fruit, releases the juices, changes the texture and flavors, and releases the smell. It's sort of like tenderizing meat, but for fruit. My granny would mix strawberries, peaches, or blueberries with sugar in a big metal bowl then serve it mixed with cream. Yum yum.
Because maceration softens fruit, it's a great last resort if you find that the fruit you bought isn't quite ripe yet.