What causes your engine rpm to raise when hot?

The increased temperature causes the air-fuel mixture to expand, resulting in a higher intake volume. Additionally, when the combustion occurs at higher temperatures, due to heat expansion, it may also result in increased power production during combustion, leading to raised rpm levels. It should also be noted that, if there are cooling and/or lubrication-related issues due to elevated temperature, higher levels of rpm than desired by default settings could ensue. Additionally, some engines utilize Variable Valve Timing technologies which change the intake valve timing so that the engine makes more peak power in a limited engine rpm range when hot.