First of all, changing to a back five is not inherently negative or defensive. City have won, playing attacking football, using that shape before. However, when making changes to counteract an opposition’s set-up, it may be perceived as a defensive change both by supporters, and most importantly, by the players.
When the shape is changed to counteract an opponent, there is probably an even greater tendency to sit deeper. It feels like a defensive tactic, and that is likely to have an impact on players' mindsets.