Why is my ceiling cracking?

It's hard to say without really inspecting it. It could be water damage from above, but then you'd usually notice some water stains as well. Could also be from the house shifting, not enough support in the ceiling if you have a second story above it or just a bad drywall job.

foundation settling

Houses never stop settling. Most settling happens within a few years of construction but not all. For example, a big tree root seeking water could disturb the soil near the foundation and make it unstable at any time. Nearby construction could also increase settling in an older home. But since the cracking occurred after the ceiling change it probably has something to do with how that job was done. Not sure what you mean by "wiling." Were the ceilings changed in every room? And do you see cracks in different rooms or is it concentrated in just one area?

Ceilings are SO difficult to finish correctly, so that is why they are normally textured in some way. I wish popcorn ceilings would make a comeback - it hides so many imperfections. To fix the problem (after making sure there is no underlying issues), texture that sucker in some way! It doesn't have to be popcorn. There are many popular ways to do this (just not as easy).

Wondering during the popcorn removal process if the joint tape was removed? If so you may need to scrape away enough of the knockdown on the joints to reapply tape and build up enough joint compound to close the joints. You can then Apply the knockdown again to the joints. I know from personal experience its not easy to match a knockdown finish. I had to remove sections of wall for an inspection. I used a knockdown sponge purchased on Amazon and it gave a passable result after painting. If I point it out to people then they see a difference. Good luck