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.)
It goes higher than YouTube. It goes way up to the highest echelons of Alphabet itself.