I've seen the big blob of immigrants before. I think that what happens is that the pops see that there is employment in some far off place, and that there is huge unemployment here, so all of the umemployed go there, even though there are more of them than there are jobs there.
This happened the day after I made the Panama Canal via having Columbia in my sphere, while there was a lot of unemployment everywhere. In one day zillions of colonists. They immediately sat there unemployed and slowly immigrated out.