I have a Panasonic microwave that keeps throwing a 20A breaker. The
data plate on the microwave says it should draw 12.7A max, and an amp
clamp confirms that it's pulling 21A

What could cause this? Is it likely to be fixable by an amateur, or is
the microwave history? It's only about a year and a half old, but it's
been heavily used in an office setting. (maybe it's time to go with a
commercial rated unit)