The leaked ROM of Tears of the Kingdom and a .nfo that specifically told people to use Yuzu
The developers recently released an Android version that works really, really well. There are many Android based gaming handhelds on the market now, and several are powerful enough to run Switch games using Native Code Execution (think virtual machine for ARM). Switch emulation was being used as a benchmark in techie videos of these products! TotK still isn't great on the most powerful ones, but give it another year or two, and you'll get the full Switch experience without a Switch.
The developers fucking sold the emulator. Sure, it was only for early access, but a sale is a sale. Then there the whole patreon thing.

Those three things combined put Yuzu right in Nintendo's sights