I think many flit in and out for a while before jumping not sure how many return permanently. I think the WT does everything possible to prevent you getting a sense of real "world" life. Its all very much like the film 'the village" .
In terms of why when people leave they would appear to hit the skids I think its 2 main things- firstly you only tend to hear of the bad cases not the ones who just go on to lead normal lives as there's nothing interesting about those. But secondly something not yet raised I think I'd that JWs are brought up with at best skewed more likely no internal moral compass. Absolutely every decision is arrived at and evaluated via JW glasses. Even the most basic societal things- death of a loved one for example _ reaction is are they a JW if not well God would have destroyed them anyway, this just isn't normal. Every decision whether its clothing, sex, entertainment is led by what the organization finds acceptable. A person never learns by their mistakes or their successes in fact learning through life is something to be discouraged and feared.
Put this together with a warped perception of what worldly people do, drink smoke take drugs and shag as many people as possible its a recipe for failure, then when it all goes wrong it just goes to show the WT was right and God is punishing your sin rather than accepting consequences for actions just as we all have to.