The answer to 'why is my dimmer switch dimming lights' is that the dimmer switch may be malfunctioning or not compatible with the type of light bulbs installed. Dimmer switches work by reducing the amount of electrical current flowing to the lights, which causes them to dim. Common issues include a dimmer switch that is worn out, not rated for the wattage of the bulbs, or incompatible with LED or CFL bulbs.
If the dimmer switch is not working properly, the lights may dim unevenly, flicker, or turn off completely. In some cases, the dimmer switch itself may be getting hot. These are all signs that the dimmer switch needs to be replaced or the bulbs may need to be changed to ones that are compatible with the dimmer.