What are they? Edit
"Dev Raids" is the term given to special raids where the target is not another player, but a developer of the game; they can be identified by the "YTYMK" prefix on their name. These "players" are built into the game and exist to ensure all features are available at all times.
The game assigns you dev raids when there are not enough other valid targets.
Why should I care? Edit
Dev raids, being built into the game, typically provide substantial rewards for a successful raid that are never depleted, as they are not actual players, and so cannot lose money. In addition, there is no threat of a revenge-raid from them, so you can plunder to your heart's content.
How do I get them? Edit
At launch, this was accomplished by simply being Diamond-grade in TDM. As more people have entered diamond grade, the only known remaining factor is your player rank -- at high rank (Maximum level fighters, highest-available grade fighters) the number of valid targets is reduced, and dev raids are more likely to be available.