Immigrants are typically more susceptible to cult indoctrination. They're a fish out of water, looking for something stable and along comes a couple "well dressed" (in cheap suits) folks who are nice to them. Of course they're going to talk to them and go to a meeting, why wouldn't they? What else do they have going on? They don't know anyone here, and this is a way to make friends.
Immigrants also tend to be a little more family oriented from what I've observed, and it makes sense that they would be. This means that when one becomes a JW, they're more likely to pull in their entire family.
I know in my area, the only congregations that I can remember having any significant growth in the last 15 years where foreign language congregations. Even those seem to have dropped off lately though. It's probably been about 20 years since an english congregation has split in my area, but the number of foreign language congregations approximately doubled (maybe more) over that time.