The part about it tripping for 8 months without you adding it up right really hits home. I had the same thing in my shed, a 15 amp breaker with a little beer fridge, a shop vac, and a space heater in winter. The fridge only pulls a few amps when it is running, but the compressor kick on spikes way higher, and if the freezer happens to cycle at the same time you are cutting, that breaker is done. What saved me was just writing down what is on each circuit and adding up the running amps, not the surge. A miter saw alone can pull 12 to 15 amps on startup depending on the size, so you were over before the fridge even kicked in. Move the freezer to its own 20 amp line and you will stop chasing this problem.