Most of it is floating around the fediverse, but a quick rundown from my superficially gleaned view:
Someone called out LTT for shitty quality videos, LTT responded publicly in a roundabout snarky way. Then a former employee called out LTT for their shitty treatment of employees, which LTT responded in an even more snarky way. Their subscriber count took a hit, and it's all been going downhill since then.
I'm sure someone will come along with more details, but that's about the jist of it.
Much worse than that - they were loaned the only working prototype of a small two-man team, along with the GPU it was designed to work with. They "lost" the GPU, tested the prototype on a GPU it was never meant to work with, publicly shit on the product REALLY hard when it didn't work, were asked for the prototype back as, again, only working copy, gave the owners the run around for a long time, then finally admitted that they'd not just sold it but auctioned it off. Very likely to a competitor of the owners.
There's also a whole other thing about workplace harassment of a former employee (who quit for mental health reasons), but I don't have the link to that right now.
Edit: The Verge have put out a really good run down of all the different issues here: