Mitch gets tired of bench-warming.
Every player, professional or not, wants to play with his team in special occasions and not only meaningless scrims or trains. This is exactly what was happening with Mitch, he didn't get to play important games and for that reason he lost his willingness to play for Trtibal.int.
The example mentioned by their note was during EDC when Tribal.int played with Azen as stand-in even when Mitch was available. This fact itself explains a lot of his reasons to leave them. He currently has no team and what he is doing next is unknown.
Tribal Gaming International roster:
Marko "Baja" Jovanovic (leader)
Dominik "Lacoste" Stipic
Amel "PlaymatE" Barudzija
Branko "Bane" Boskovic
Gordan "ninja" Prosic
Links
Tribal Gaming - Source