Kids leave soccer for the same reasons why they stop playing an instrument or drawing or reading for pleasure or listening to parents . . . they become teens and make their own decisions about what's fun, what's worth their time, what's worth the effort and what's more valuable to them (eg, listening to parents or listening to peers). I think parents find the change of gears from their athletic kid so jarring because parents become so immersed in that life that the kid saying, "enough" (especially if they are talented), seemingly comes out of no where and we wonder/worry that it is not a reasoned decision. We might ask ourselves, "Do they really know what they are giving up?" but the question can/should also turn back at us for the kids that continue to make a focus of their life soccer (or any other consuming activity), "Do WE really know what they are giving up by staying in the sport?"
I think
@crush's comment is so interesting - the one about the kid saying she's quitting and she has a boyfriend. That parent may have been dumbfounded but I'm guessing that if the signs were not overtly there, this kid has been less in love the sport and her place in it than the dad had been for quite a while. Unless there is some terrible episode - a kid or adult being particularly cruel, for example - these decisions are rarely spur of the moment.
(I've been a parent for nearly 27 years. My 4 kids all played sports growing up - my son stopped in HS and my 3 daughters still play at age 23, almost 19 and almost 19. I think I was good at letting them chart their own paths but I know I could have been better and often wonder just how different I may have approached the journey if I were going through it w/the knowledge that experience brings)