What are the criteria for pop decline?

[Vielmann]

I noticed that sometimes in the pop growth screen a pop is shown as declining for several months and then stopped. Also, when I play life seeded, I can't declare population controls on my non gaia planets without causing pop decline (I want to cause emigration push, so I don't have to resettle all pops while my robots keep being manufactured).

 

[Methone]

I can't make heads or tails of it, but the wiki at https://stellaris.paradoxwikis.com/Population has some stuff about it. Maybe you'll fare better than I did.