Degentrification is when an area or neighborhood starts to gentrify so quickly that it does not 'organically' gentrify and certain elements of that process break down and begin to erode the process. (meaning, the process is too rapid to have ever happened over time, culturally and of its own accord.) In the case the north brooklyn, the gentrification has almost been forced or pushed by condo builders in order to sell condos offering incentives that they never had any intention of producing. When this happens, word spreads fast and those reasons which originally inspired people to move there become reversed and disappear.
Take a look at all of the condo buildings that have gone rental and you'll see degentrification at work.