To be clear: the Nintendo Switch is not burning MicroSD cards. But: with 1.5 million consoles sold and climbing, the market demand for high-density MicroSD cards have shot up, and in doing so has revealed a number of flaws with available supplies. In particular, SanDisk's 128GB units heat up to the point that the plastic melts.
Again, this isn't unique to the Switch. As Amazon customers have reported, the SanDisk model, one of the "first 128GB MicroSD cards" verified for GoPro usage, is melting in laptop SD ports too.
But since the Nintendo Switch only has 32GB onboard memory space, owners are looking for solutions to expand their download capacities. Upcoming title Dragon Quest Heroes, for instance, will take up the full 32GB, which is technically more than the available storage space thanks to the need for OS storage, save files, etc.
It is not yet known if other MicroSD cards or brands have similar problems (the writer's own 64GB card doesn't seem to have turned to slag yet). The Nintendo Switch has theoretical compatibility with MicroSD cards for up to 2 terabyte storage -- not that such cards are actually available on the market yet.
Clarification: As the author of the original tweet's blogged to re-emphasize, the issue is not with the Switch itself, but defects with the MicroSD card in question.