Apart from the part at the top of the blog post which gives some context; Essentially Marty Stratton (id Software Studio Director) behaved extremely unprofessionally towards Mick, publicly smearing his name. He lied repeatedly, posted the Reddit thread after the two agreed to not post anything and to make a statement later amicably, and much more.

He put him under a massive time crunch (sleeping in studio over Easter and more), didn't pay him for like half his work, replaced his work on the OST with Lead Audio Designer Chad Mossholder's made of edited chunks of his in-game score, they'd been working on, without telling him for six months, which was not up to scratch (you can hear some here: https://www.youtube.com/watch?v=BbhWmLVAvJw) didn't pay him, and tried to shut him up with a six figure settlement and a gag order.

That's just the tip of the iceberg but you can tell how bad it is.