The reason why the protagonist is or becomes involved in the plot is left unexplained, effectively functioning as a sort of Deus Ex Machina.
Alternate name: Undefined involvement
The first video game about Inexplicable involvement was released on June 24, 2003.
e.p.i.c. interactive entertainment has published all these games
This is also useful in cases where player accepts quests/work/etc. without clear reason (as opposed to say working for money).
This may be there simply to allow players themselves to supply the reason, essentially making it an instance of obscured identity.