It was really unexpected, how could Manchester City lose so badly with a score of 5-1 like that.
Arsenal through two latest seasons competing for Premier League title directly with Manchester City, in several matches show their strength and ability to beat Manchester City, but failed to have a win directly to this strong opponent. This season is a time for Arsenal to win against Manchester City, and more impressively they got a landslide win.
I believe their backline has been weak since months ago, hence why they bought a new defender although it seems like he needs time to settle in. He's very young, to begin with, so putting too much pressure on him makes no sense. It doesn't help that their midfield is out of depth too. They made so many errors that Arsenal could've scored 7 goals IMO. The players' mentality is so low that they made terrible passes on their defense, leading to so many counter-attack opportunities from their opponents. This stuff happened when they faced Chelsea too iirc.
Arsenal exploited Manchester City defensive weakness very well and abused the opponent's mentality drop very efficiently. They deserved this big win because they played well and also because of terrible mentality and performance of Manchester City. This defeat again raises an alarm on Manchester City, that they returned to top four, they escaped Champions League group stage to playoff stage but if they play like this when meeting Real Madrid, they will have to go home soon.
In Premier League, they have many things to do for completing this season in top four, with many teams are joining top four competition.