What is it that makes these people return to the Borg after leaving for years?
It depends on the reasons why you left in the first place. Someone who left for doctrinal disagreements will most likely not go back, because he knows there's nothing of value in what the WTS offers. Now if someone left for sexual issues (like I did, following the majority I think), and they don't find "the truth about the truth", they will probably go back. The everlasting-life-in-paradise thing, as opposed to eternal death, is a very convincing reason to go back if you still believe it.
Personally, I had been out for 4 years and still believed it was "the Truth", until I accidentally discovered Randy's homepage. During those 4 years I lived a perfectly happy life, but I was completely uninterested in religion. Or more to the point, I was completely convinced that the JW had the truth and there was nothing of value outside (religiously speaking), so I didn't bother looking for something different.
It's amazing, now that I look back, how powerful mind control is, especially when you were raised a JW.