In one of my cities I had a population of about 30k. Without really doing anything it went down to about 15k in a few minutes and I have no idea why. Overall happiness and approval rating is increasing and residential demand is full. I dont get it. I increased my land value and saw a few high rises pop up and my population went back up to 20k. My city is a lot denser now and I have about 30k people, about as much as I had without any high rises. Any ideas?