Someone answered to me globally here, but if someone has more details, feel free to add :)
I'm quoting him totally here :

(quoted from Epitaph666's posts) :

"Won't bother finding the exact quote, but the story starts after Stainless had finished Carmageddon 2. A game that was also rushed out to be released (remember the "Digga" vehicle by the driver Ron Dumpster? If you picked that car the game would crash. It was because the game was rushed and they couldn't playtest it).
So after the release of C2, SCI (their publisher) asked Stainless to make a third game. Stainless replied that they were too burned out, and they wanted a month (or sth) off to recharge their batteries. SCI said "ok no problem", but they secretly asked another dev (Torus games) to start developing the TDR 2000. I don't remember if Stainless got pissed off when they found out and left or SCI just ditched them."

"In short Stainless Games invented and created Carmageddon, but back in the 90's you needed a publisher. SCI stepped up and they acquired the rights to Carmageddon. But then SCI fucked Stainless and secretly told another developer to create the third game. Stainless (the original creators) had lost the rights for 12 years until 2011 when they reacquired the rights for the Carma brand and kickstarted the series. So there's pretty strong feelings for TDR 2000 as you understand."

Thank you very much for the link :) (and on the release thread ;))

It's even a bit worse than just what has been explained so far. Not only did SCi put another team on the Carma3 project, but they also took the first opportunity to stop paying Stainless while they were working on their next game (Arena AD). Not only was Arena AD canned, but Stainless had to merge with VIS Entertainment.

Really not a nice story for Stainless :/
Thanks for the additionnal explanations !