you have three nervous systems, the Central Nervous System which we dont really care about here
but the other two which we do care about here are the parasympathetic, and the sympathetic nervous systems.
you basically have one brain circuitry in ur body for resting n bein chill (parasympathetic nervous system) and u got another brain circuitry for running about and kickin hella wooly mammoths and goin into beserker mode (the sympathetic nervous system). you can't really be running both at full pelt at one time (i.e. you can only play one version of pokemon on ur 3ds at any one time) and ur concentration end of things is in the Chill part (naturally) and not the Unchill part.
going into them is too complex for a small carepost on a pokemon website but in an extremely handwavy sense your nervous system/brain uses brain chemicals or "neurochemicals" to do many things in the body, and one of them is to switch from one of these aforementioned nervous systems to the other.
(in the tortured videogame reference imagine the neurochemicals are you switchin the videogame over when you want to play Pokemon Parasympathetic instead of Pokemon Sympathetic. in this example Pokemon Para. is the chill game with the chill evil team and Pokemon Sym. is the one where everyone is SCREAMING and u get like running boots 2x and can't go into doors cos ur too busy crashing into the walls next to the door cos ur running too quick)
the neurochemicals that govern concentration and being Calm after panic (norepinephrine) can be offset by the brain chemicals associated with Stress and Anger (epinephrine and serotonin respectively) when ur brain gets put into a bit of a Stress playin the vidya game.
stress/anger brain chemicals go up, and ur brain switched, and neurologically ur fighting a wolly mammoth in order to survive. but not really, you're havin a tantrum cos you got brutally owned in a videogame and all this pent up rage has to go SOMEWHERE. and then after the tantrum ur neurology goes "ok we're safe and not gonna die now" and ur norepinephrine kicks back in and you switch back to the Parasympathetic nervous system (the chill one)
additionally if you are genetically/neurologically....uh....Different (so to speak) i.e. you have adhd, add, autism, bipolar, unipolar depression, multi-ass, suicidal tendancies discography, conclave chest etc etc you are kind of predisposed to have a different level of neurochemicals for reasons scientists dont quite understand yet (neurology is a complex thing) so you're even more prone to losing your concentration in hilarous and illogical ways when getting mad at a videogame for Complicated Neurotransmitter Reasons