I've no idea how it happened in me. I've asked my brother, who says he has no issue visualizing things. My parents are gone, so I can't ask them, but my mom was a visual artist who was clearly able to switch how things looked in her 'inner vision' and then create a painting / watercolor / sketch to match. My dad an amateur photographer, who was focused on the technical aspects, so may have had it (I'm a photographer, too - with similar focus - so it's not necessary for that). I may have inherited it from him, but I suspect that it has to do with my having really bad vision, but it not being corrected until I was in fourth grade (guessing I was smart enough to compensate / cover up until then). Anything similar in your kid's history?
All that said, it hasn't been a handicap at *all*. It's pushed me to be very verbal, learn things from first principles (I suck at rote memorization, which may or may not be related).