Why would a motor capacitor keep dying? What's the usual culprit?

Honestly, I think you are onto the right track with the motor drawing too much power, or at least somthing is stressing it. I had a similar issue a few years ago with a Bosch condenser dryer, and the capacitors kept failing even though they were good quality ones. It turned out the issue wasn't the parts themselves but the voltage coming into my house, which was fluctuating way outside the normal range for that part of the city.

When a motor struggles, even if the physical drum is free turning, it creates a huge load during the startup phase. If that load is consistently high, the capacitor, which is basially just a temporary battery for the start of the spin, gets overheated and stressed out way faster than expected. The heat is a symptom, but the underlying cause is likely mechanical stress or electrical instability. If you want to check this out without tearing the whole unit apart again, maybe have an electrician check the power supply to the appliance. Or, if you want to keep it DIY, try swapping the motor for a compatible one from a diffrent machine, just to see if the heat pump logic handles the load differently. It's not worth spending money on replacements if the fundamental electrical setup is fighting you. That machine is probaly fighting the power grid, not you.

I'd push back a bit on the voltage fluctuation idea, even though it's easy to throw out as the culprit. From what I've seen, with these high-demand cycles, the thermal stress is usally the real killer. Heat pumps rely on precise expansion and contraction of these components, and if the surrounding area in the utility room is warm, you're essentially baking them. I had a similar situation with a Whirlpool dryer, and the replacement capacitors started failing after only a couple of months becasue the dryer was stacked right next to a furnace, making the back panel consistently around 115 degrees. The heat causes a slow degradation in the dielectric material of the capacitor, leading to premature failure, regardless of how high the starting current is.

It's less about a sudden overload and more about accelerated aging from constant temperture exposure. If you can't provide proper ventilation around the unit, or if the drum is constantly over-filling and forcing the motor to run longer cycles than designed, you are guaranteeing premature component death. You should probably try to see if there is adequate airflow behind the unit, expecially near the coils. Stopping repairs when it starts fighting you is hard, but if the enviornment isn't helping, it's just a matter of time before the system fails completley.

First post here so bear with me. When my old washing machine ate two caps in a row I figured the motor was on its way out, but it turned out my first replacement wasn't the same rating as the original. Ran fine for a bit then died the same way. Second time I matched the rating exactly and never had the problem again. Maybe worth comparing the marking on your new ones against the dead originals before blaming the machine? Also, and I'm really guessing here, is there a chance the little switch inside the motor that's supposed to take the cap out of circuit once it's spinning is sticking? No idea if these dryer motors even have that but it woud explain a cap cooking slowly.

I had the same never-ending replacement cycle on my old dryer and it turned out fluff was packed around the motor, the bit that spins the drum. Cleaned it out with a vacuum and the brush attachment and the problem stopped. Even if the drum turns freely by hand, all that lint makes the motor run warm and I think that warmth is what slowly cooks the little starting part inside. If yours is a few years old and you have never pulled the fluff tray out and vacuumed the channel behind it, that could be your answer. Does the machine seem noisier than it used to be when it runs?