Maybe your game was loading a lot of props and stuff in the background without you noticing. Opening a game changing system like the one you changed the city with can then cause a crash. If possible, please try doing this the exact way a few more times over the next days and check back here and let us know how it went
That sounds a lot like what I described in my previous reply. Changing ad screens is a lot of work for the game, just like changing all of the vegetation at once.