What's causing that to happen

That means the circuit has a problem, not necessarily the sensor. There's two; one for the Engine Computer and one for the gauge on the dash. If the dash gauge is reading incorrectly, check the single-wire sensor. You need a scanner to display the live data the Engine Computer is seeing to tell if the two-wire sensor has a problem. A connector could be unplugged, a terminal in the connector could be spread or corroded, or a wire could be cut or corroded apart.

So if I understand right you're talking about the dash gauge reading too high indicating the engine is overheating, but no engine can get that hot so quickly. The place to start is by connecting a scanner that can display live data to see what the actual temperature is according to the Engine Computer. If it's high, diagnose the fan and cooling system. I suspect it will be normal because an overheating engine won't cause the warning message to appear.

There's a problem in the sensor circuit, not necessarily with the sensor itself. If we're still talking about the gauge reading that high, the engine is not really overheating. Unplug the sensor, then measure the voltage on the wire with the ignition switch in the "run" position. See what the gauge reads. Next, use a jumper wire to ground that wire, then see what the gauge reads.