This happen to me multiple times. For example, there were 3 players. One had a metro, the other two(including me) had one city each. So there were 3 cities in total.
I activated one knight and no one else did. My city was NOT suppose to fall since i have the most number of Knights but it did. Both of us who didn’t have a metro; lost a city.
It is clearly stated in the rules that the player with the lowest Knights, apart from having a metro, loses a city.
This only happens in the beginning of the game. When players have more cities and knights, it works fine.