Japanese repair shops are apparently getting quite a few customers who are experience issues with their Switch overheating. Some stores are now out to spread the word on what's causing these issues in order to prevent future problems.
It seems the problem stems from the HAC-CPU-21 chip, which is connected to the Switch's fan. A glitch prevents the fan from turning on, and thus, your Switch will overheat. In order to remedy the issue, you’ll have to remove the HAC-CPU-21 chip to get the fan functioning again. That said, it's not guaranteed the problem won't rear its head again.