75C isn't high temperature at all.
While I agree that such an increase under that condition is peculiar, it shouldn't be worrisome.
I'm worried about the fans getting used up much faster for no apparent reason. I'm curious if others have encountered something similar and whether or not they found a solution.
Whats happening is your GPU is lightening its processing load on the mining, and trying to balance out for the video rendering... this in turn, is making it not work quite as hard as if it were just mining. I know AMD's processors are arrange differently than Nvidia, so this is the first i've heard of this type of thing personally. I have a machine the mines that I use daily, and even game on while its mining. I can even run a nvcaputre recording program at the same time as well. I get about 1/3-1/2 the hashrate that I normally get when not playing games.
Anything under 80* is fine honestly; I typically (during hot summers) put my GPU's at a fixed 75% fan speed and the GPU's usually hover at an acceptable level.