I poured simple syrup over cooked apples. Why did it turn watery?
Some of the water content of the apples has migrated from the apples to the surrounding syrup.
This is due to an effect called osmosis. The apple, like all living things is made out of cells that have (among other things) water inside them. The cell walls are semipermeable, meaning small molecules like water can pass through them, but larger molecules like sugars cannot.
All dissolved substances (sugars, salt, etc.) attract water to themselves, so if you expose the apples to a highly sugary solution (like syrup), the water will be pulled from the apples into the syrup.