Jeeze! I've never seen something like that before! Are you sure the rad is getting adequate air flow?
Also, it could be that the GPUs are heating that water to 75c. They could still be at 50c even though their output water is at 75c.
As i mentioned in an earlier post, I can have the GPU's idling or running Furmark, GPU temp will rise 10c from ~40c to 50c, but the CPU will do exactly the same thing: Cores rising fast to ~39c and staying put, CPUTIN will slowly reach 75c and pc will shut off. I can run stock clock and vcore and it'll reach around 67-69c CPUTIN.
More pics of the install here: