It's the way they design the tracks. They invariably put a trampoline-like substance in that area just coming off the curve which launches you toward outrageous 100 meter splits for the latter half of the 200. I am sorry to have to be the one to spill this secret.
its because you've had time to get your momentum going and the second 100m is straight as opposed to curved. you run faster in a straight line, theres a reason you dont see any 100m races run on the curves (barring 4x100m)you also are able to really get your legs moving underneath you as you no longer have to control your speed to stay in the lane as you do on the curve. try sprinting full speed and then turning while trying to maintain full speed.
Same reason why straight 200m races are faster than traditional ones.
You lose speed maintaining the curve. a percentage is spent on centripetal work / acceleration making the curve. on the straight 100% that work is spent pushing your feet backwards propelling your body forward.