The answer to 'why is my switch dimming lights' is that the dimmer switch is likely failing or not compatible with the light bulbs. Dimmer switches work by reducing the amount of power delivered to the lights, causing them to dim. Over time, the internal components of the dimmer can wear out, causing the lights to dim unevenly or flicker. Additionally, if the dimmer is not compatible with the type of light bulbs you are using, it can cause the lights to dim improperly.
Some common symptoms of a failing dimmer switch include flickering lights, lights that won't dim all the way, or lights that dim unevenly. To troubleshoot, you can try replacing the light bulbs with ones recommended for use with your specific dimmer switch. If the problem persists, it's likely time to replace the dimmer switch, which is a relatively simple DIY project. However, if you're not comfortable working with electrical components, it's best to call a licensed electrician to diagnose and repair the issue.