You see this on older marathoners too. I am personally in no position to make any claims regarding this, but if you go over to a weightlifting or bodybuilding forum and ask the same question they will respond that belly fat on otherwise fit people doing mostly aerobics is due to an over-reliance on low intensity aerobics, too little muscle mass in the upper body, and not enough anaerobic training.
The cure: do intervals and other lactic acid-heavy type training and pump iron regularly off the bike. And not 20-rep sets, that's just more aerobics.