Those bright red, blue, or white dots are most likely hot or dead pixels on the sensor. They can exist all the time, but they become much more noticeable during long exposures, especially in dark scenes.

This is normal enough that many cameras include a long exposure noise reduction feature. When enabled, the camera takes a second exposure of the same length with the shutter closed, identifies pixels that still show up bright when they should be black, and removes them from the final image.

So the cause is sensor pixel defects or pixels that become very visible under long exposure conditions, not something in the scene itself.