Do you time travel? Maybe that would have some kind of effect but otherwise it's completely random. No need to worry. Like superblooper mentioned, you can get a villager in boxes from someone if you go to the villager trading board. I have Bam moving out in a couple days, too, if you want him.
If you have 8 villagers, no-one will be able to move until you get a 9th one. The game will eventually give you a random 9th villager, or you can trade for one/get one from streetpass!