I had just climbed up on my roof to stain some siding, having already tested the paint sprayer (Wagner) to make sure it worked. When I got ready to use it, I pulled the trigger and...nothing. Well *@#$&&#$&, had to climb back down to check things. Turns out the GFCI outlet in the garage was tripping. What the heck? Maybe the outlet had gone bad? Nope, tested with a drill motor, works fine. Ended up getting a new sprayer and finished the staining job. Then I tore into the old sprayer to figure out what could have gone wrong. Tested for continuity with multimeter at the plug, good, tested at the switch, good. Didnt want to tear into the motor housing so that left me wondering - what would cause the motor the trip the GFCI like this? Isnt't generally a short that causes a GFCI to trip?
Start with: "What does a GFCI trip on?"
A GFCI is looking for GF's, or Ground Faults.
To be more precise, it is comparing current on the hot wire vs. current on the neutral wire. If those currents are different, that means current is leaking via some third path - through you to the earth shocking you, for instance.
The trip threshold is 5mA or 5/1000 of an amp. That is because 1mA is noticeable and 10mA stuns. "A stun is as good as a kill" if you fall off a ladder or end up face down in water. That's particularly important at dockside - freshwater electrocutions not only take out the victim, but the first rescuer or two.
So you're not entirely wrong that it's a short inside the machine, however it's not much of a short. It is only modest leakage, which can be caused by a variety of damage or contaminants. For instance worn motor brushes - where does that brush material go? It winds up as dust inside the motor. A coating of conductive dust inside a motor could trip a GFCI. So could many other worn parts.
Note that a "neutral to ground fault" is also a ground fault, and that's important because sockets are sometimes wired reversed or many plugs can be put in "upside down".
Have a look at your extension cord. Check every inch of it for cuts, breaks or exposed copper. For your first trip up the ladder, a break in insulation could have been touching damp ground, vegetation, the ladder or any other path to ground. The second time up when you had a new sprayer, the break (if there is one) could have been in a different random position and not leaking current to ground.