I don't quite know unfortunately -- what I can tell you is that something is eating up all your CPU. OBS itself isn't using that much CPU, because it's only encoding a 720p 25fps video, and you don't even have any other things active like webcams, so OBS is definitely not eating much CPU itself. Something else is eating your CPU. It could be WoW, or something else on your computer doing that.
Alternatively, if you can't figure out what's going on, you could just use a bit more downscale in OBS to reduce CPU usage a bit more if you can't figure out what it is. You could also try "superfast" x264 CPU preset if you don't want to modify the resolution, though that will decrease your quality a bit in exchange for decreasing CPU usage.
Again, your CPU isn't that bad so it shouldn't have too hard of a time handling it, but I don't know how much CPU world of warcraft uses these days so I couldn't say.
