Ascended Fanon: Originally, there were rumors circulating around the Internet that one of the higher-ups at Nickelodeon sent the okay for the production of new seasons of several old 90s cartoon shows, including Rugrats, Hey Arnold!, Rocket Power, Doug, and several others. Though false, the rumors generated such a buzz that the execs did pick up on it and decided to create the late night block for TeenNick.
Within the block, CatDog seems to be getting just a little bit more attention in advertisements than other Nicktoons.
Some have accused Hey Arnold! of becoming this for the block. While most of the cartoons shown from 10:00PM-11:00PM are supposed to rotate each week, Hey Arnold! seems to fill this slot every other week, and whenever the block has time to fill, expect to see Hey Arnold! there.
Nick finally takes advantage of their legal right to show pre-sale-to-Disney episodes of Doug.
While Nick has the airing rights to the whole Power Rangers franchise, most people probably weren't expecting Mighty Morphin' Power Rangers to air in any capacity on the block. It ended up airing as part of the "Mighty Morphin' Weekend" event to celebrate the 20th anniversary of the show. No clue if it'll return along with the rest of the 90s Power Ranger series.
Edited for Syndication: One of the drawbacks of airing 1990s reruns of kids' shows 20-odd years later. TeenNick cut out the musical performances from All That due to music licensing issues, and Rocko's Modern Life is still shown edited since Nickelodeon lost the original masters at some point. (Similarly, these versions are also on the Rocko DVDs.) Which becomes Fridge Logic, since the original masters are available on Nick in Canada. All the American Nickelodeon has to do is ask Canada nicely for the masters (hey, they already hijacked You Can't Do That on Television for themselves, so why not?)
Disney launched a YouTube channel showing their old work (Not literally a block, but a channel to watch the old shows as well as new ones too.), and later, brought a similar block for Disney Channel... but on Thursdays only. (And after that, release a few blocks of classic shorts to Netflix.)