why does the water go hot all of a sudden?

This is usually caused by a fluctuating water pressure due to someone else using a large amount of water all of a sudden, even if it's someone in a different apartment. When you take a warm shower, you have the hot and cold water on, with both mixing in the pipe that feeds the shower head. If you were to suddenly reduce the pressure of the cold water, the reduced cold water flow would make the water hot momentarily until the pressure equalized out again.

This is usually caused by someone flushing a toilet, where the supply valve is suddenly thrown completely open, reducing the pressure in the water main that supplies the building. Even if it's another unit in the building.

It can also be a leaking toilet flapper valve causing a toilet tank to leak into the bowl. As the water level in the tank gets low enough, this causes the fill valve to pop open to refill the tank, as though someone flushed the toilet. You then get the momentary reduction in cold water pressure, resulting in a slug of hot water out of the shower head.

Water softener operation?

Water closet flapper leaking?

Refrigerator ice maker operation?

Lawn sprinkler system operation?

Somewhere cold water is running intermittently?

Is there a toilet in the house that runs every no and then?

maybe your house is haunted