David Alaba is Bayern Munich's leftback. Which naturally means that he's in the middle of the field playing as an attacking midfielder and pressing Arsenal in the middle of an attacking sequence, completely independent of his defensive duties.
This is just what normal soccer players do. It's not at all simultaneously wonderful, terrifying, exhilirating, and completely insane. Which coincidentally are all adjectives that describe what he did directly thereafter.
He presses the ball and wins it from two Arsenal players, takes a dribble, and then launches it perfectly over the head of a completely unsuspecting and frantically backpedaling Petr Cech in a sequence that barely lasts 3 seconds.
Absolutely nothing about this Bayern Munich goal makes any sense except for the fact that Pep Guardiola is completely insane himself and David Alaba is his instrument for world domination.