Because it is not securely fastened.
The gauge does not cause the heat; it only shows what it is.
Idle the engine, remove the temp sensor wire on the engine and ground it. If the gauge does not move one can assume it has failed.
replace temperature gauge
Fuse location for temperature gauge for a Malibu
Defective temperature gauge or defective temperature sensor?
Yes, they move slowly but constantly. Faster during earthquakes.
The temperature gauge sending unit might be bad. To check it find the sending unit on the side of the engine block (on most models) and pull the wire off and ground it with a piece of wire and have someone look at the gauge to see if it moves.
why is the temperature gauge not moving on a 2007 Lincoln mkz
Bad gauge or defective coolant temperature sensor
Why does your temperature gauge rise and lower only when you have the heat on?
An automobile temperature gauge can fluctuate when the thermostat is opening and closing erratically. The temperature gauge can also fluctuate when the ground wire is loose.
The thermometer that reads your engine temperature, I presume, is damaged or broken. Without the gauge to tell your fan how hot your engine is it will keep running as a 'better be safe then sorry' feature.