Hot pixels are usually caused by a pixel generating extra electrical charge even when little or no light is hitting it. The camera’s electronics then interpret that charge as if the pixel received more light, so it shows up as an abnormally bright dot.

The main causes mentioned are:

• small manufacturing variations between photodiodes and transistors, so not every pixel behaves identically
• leakage current, where charge leaks into a pixel well and is read as signal
• damage or changes from radiation, which can affect individual pixels and make a formerly normal pixel become “hot”

In practical terms, a hot pixel is often a pixel whose dark current or leakage has become unusually high. This can happen from the start due to normal sensor variation, or it can develop later if the pixel’s electronics are altered or damaged.

So a “good” pixel becoming hot usually means its electrical characteristics changed, not that it suddenly became more light-sensitive in a useful way. It is producing false signal.