Mat worked in the fledgling gas industry in Chernarus before the outbreak and ensuing conflict broke out and isolated the area, he tried to flee back to Europe via the Novigrad airport with his co-worked Dan but they got there too late to join his work colleagues, some of whom may have made it, some most likely didn't.
Working remotely in Berezkhi at the time didn't help, having no means of transport made it much worse. In the turmoil that followed survival became an increasingly bleak prospect, especially after Dan became infected.
Mat tried to nurse him back to health, thinking he had a simple infection, when Dan seemingly turned rabid and attacked him the ensuing struggle resulted in his friend falling to his death off coastal cliffs.
When Dan simply stood up after enduring a fall that would kill any man, continuing to rage incoherently, that's when Mat start to fully realise the unprecedented horror that was now shaping his world. After returning every day to 'check' on his friend, seeing him decay yet seemingly still alive, he started to doubt his own sanity at what he was seeing.
Eventually he left his friend to his immortal living hell and began to wander the plains of Chernarus, still hoping that he could somehow escape and return to a normal life.