I have a question about moving villagers, but I haven't seen a clear answer...
I know it is very possible for a villager to move without warning/pinging you about it first. This has happened to four of my friends as well as myself, but luckily for me, the villager who moved out was one that I did not interact with much at the time. However, I was never notified that he was going to move. Villagers I spoke to regularly never dropped hints that he was thinking about leaving and he never pinged me himself, even when I passed by. The only notification I ever got was when I booted up the game and Isabelle told me he was leaving.
What are the possible causes for this to occur, and is it preventable?
When you have 9 or 10 villagers, you're in the "Moving Out" period where villagers will ask you about moving out. When you don't talk to a villager for a week or more (may it be tt-ing or ignoring), they won't approach you until you approach them first which can result in unexpected move outs. But if you talk to them which triggers the "I haven't seen you for so long!" dialogue, just stay out of their site for a while or save & quit and they'll ping you.
I use this technique when I tt so I know which is the "moving villager" to prevent any villager I like from moving out.
But this is simply speaking from my observations and personal experience.