http://misterikaen.blogspot.com/2016/06/lars-mittank-mysterious-disappearance.html
This guy is from Bulgaria, he put together few articles about this case, it seems quite valuable.
I think that we have 5 options:
1. He wasn't drugged or irrational and he was followed.
2. He was drugged and irrational as nobody was following him.
3. He was drugged AND totally scared because someone was following him.
4. He wasn't drugged or scared, but he was in hurry and realised that he forgot about something - so he run to get that and anything what happened to him was unrelated to that fight or threats.
5. It was an act to disappear.
Last option seems to be very unlikely since he had no reasons to do it - and his calls and the way he acted at the airport was rather unusual, why someone who wanted to disappear would behave in a way which could bring just more attention to him?
First also doesn't feel right for me. He was at the airport, ready to leave the country, why would he run away from place which is well guarded, relatively safe, full of potential whitnesses and cameras? Even if he wasn't in the right condition for flying, running away from there into unknown, open space, where exist much higher risk for getting harmed?