Okay, they are not dropping in absolute value, but in relative value.
There is a well known pattern to gentrification, you are describing second (or perhaps third) wave gentrification. You can basically draw concentric circles around a city center and the gentrification will pulse outward in waves radiating along those circles. It is never exact because some areas hold their value better than other either because of a natural attraction, some economic reason ( for example a large business that is unusually successful), or a social one (successful ghettoism), but it is generally circular, and generally moves together.
And some areas hold their value better because they are places lots of people want to live but government bans new development in them.
