20 amp circuit breaker trips after using 15 amp miter saw
A large inductive load, such a motor starting, often causes a ground-fault circuit interrupter to trip even when there is no fault in the device. A snubber, a small capacitor and resistor across the line, can help mitigate the issue. A voltage surge protector can also help, but avoid protectors with large varistors from line to ground, which can also trigger the GFCI.

You might also need a Class C or other type of GFCI which allows greater fault current to flow for a longer time, depending on shop voltage, appliance grounding and insulation.