Where did my friends go?

I was reading the 33 Things posting on Evangelical Outpost, and noticed this little infographic:

(Click to enlarge, I think.) First, I noticed the little red snakes. Then, I noticed the bigger yellow snakes. And then, I noticed the huge green snakes.

But the yellow snakes are what intrigue me. Why do so many leave evangelical churches for mainline churches or no church at all?

(I figure the Catholic snakes might just be children who grew up in Catholic schools, for whatever reason.)