Trying to learn pvp and I came across this and simply dont understand

I was doing quest after wintergrasp, when I came across a dk who had ganked me before and it was fishing. So I went to return the favor, and Every single ability i used was immune. I am just trying to understand as to why? Here are some screen shots, Thank you for any help in understanding why!

https://imgur.com/M5E2x0q
https://imgur.com/WJyOp7s
https://imgur.com/EN5HMiG
https://imgur.com/cflIrnd

You have immunity to everything when you stand in one of the graveyards your faction is controlling. It's the yellow buff.