TL;DR: YouTube sends a Cease and Desist email to the Invidious team for API abuse (even though they don't use the API.)
I also want to point out (since nobody else has) that YT has done shenanigans like this in the past. Invidious is not going down without a fight.
UPDATE
As of 2 weeks ago, Google has begun blocking multiple instances from accessing their services. This also includes my own instance and appears to be done alongside the C&D. (Note: I have not received one yet.) I'm currently playing whack & mole with Google and will be moving between several different servers. Some others are going through IPv6 addresses (which didn't work for me.)