I'm betting a leak as that is the one weak spot of any enclosed cooling system. Those joints/clamps and rubber hose tend to give out over time. Corsair also had some quality issues as well for a few months where they were having to replace their systems like hotcakes because people would end up with leaks a couple months after installing them. Corsair then had to pay to have their motherboards and w/e it destroyed replaced. I can't remember the year though on when that happened. It was on a lot of the tech forums at one point but I know Corsair had them quietly take some of it down once the situations were resolved to the customers liking. Not shocking they would do that though most companies will in order to protect their reputation.
All in all though Corsair has been rock solid for me in the enclosed cooling systems and I've had lemme think 5 of them all ranging from the H50-H55-H75-H75 again and there was a H60 as well. This one has a refurbished H75 in it going on 3 years. Next one will be a H110 or a Fractal of similar size. Plus I always double up on the fans on the radiators, one to push air in and one to pull the air out as I have mine setup with it right next to the CPU right now vs up top. But the new tower will be interesting. Seasonic 850 Full Modular vs the Antec 750 Gaming Full Modular now, 32gb of 4000+ memory, 11th Gen Intel and w/e they come out for chipset and they better well damn have PCI 4.0 for the SSD's as I plan on running 3 of those if I can