It's probably the tank's fault.
I just had a DRK in Fractal that didn't once click a single defensive cooldown no matter how big the pull. I had to stop helping with DPS and just use every cooldown I had to keep him alive. I asked him why and his response was "I'm just too lazy to use them."
You also have to remember that DRK is the new DRG. The edge-lord job that attracts to it every single type of worse player you can think of who attempt to act like it's some kind of glorified DPS (because they secretly want it to be) and don't learn a thing about how to tank probably during their entire trip from level 30 to level 60.