In a nutshell, he owes a lot of people money, including Josh, Derrick and Darren, all of whom were fired via text message. They weren't even allowed back on the property to get their own tools. He never paid them for their work on the show even. It was all IOUs that never happened. Mark allegedly also stole parts from other customers cars to use on other cars, which created a never-ending cycle which led to lots of parts being robbed from Darren's car and others. Holly left after that short time due to issues with Mark though no one would speak to what those issues were.
There is a reason for the giant gap of episodes this year, it's because all of this fell apart around the show in the midst of filming.
This is directly from Darren himself and confirmed by Derrick at separate time. So it is 2nd hand info and heresay but that's the just of it
If you want more details, pm me. But thats short version.