Why is my breaker tripping even though I'm using a heavy-duty extension cord?

The cord is fine. Your breaker is tripping because something else is wrong — probably the tool or the breaker itself. Motors pull a startup surge of 2-3x their running current, which can spike a 10-amp tool to 25-30 amps and pop an old or sensitive breaker. Check the tool for a short or mechanical binding, swap the breaker if it’s worn out, and make sure you’re not running other loads on that circuit. Start with the tool — that’s where I’d bet.