The answer to 'why is my dimmer switch circuit overload' is that the dimmer switch is likely trying to control too many lights or a load that exceeds its capacity. Dimmer switches have a maximum wattage rating, and if the total wattage of the connected lights exceeds that rating, it can cause the circuit to overload.
Some common causes of a dimmer switch overload include having too many lights on the circuit, using bulbs with a higher wattage than the dimmer is rated for, or a faulty dimmer switch. Symptoms may include flickering or pulsing lights, the circuit breaker tripping, or the dimmer switch feeling hot to the touch. To resolve the issue, you'll need to reduce the total wattage on the circuit, replace the dimmer with one rated for a higher capacity, or call an electrician to inspect the wiring and dimmer switch.