He flipped, and being raft trainees they didn't know enough to let go of the kayak so he could roll back up. After a few tries he decides to bail out and goes to pop his spray skirt.
The previous year, having gotten tired of always getting water into his boat, he splurged on a new sprayskirt that had a rubber sealing strip instead of the usual bungee cord. That rubber strip had now lost its elasticity in the cold and had frozen to his kayak (did I mention it was snowing?). When the guides in a raft saw he wasn't coming up they got to him and dragged him, boat and all, into the raft. He was pretty blue, but still breathing. A quick fire to thaw him out, a stern lecture about kayak rescue protocol to the trainees, and they were back on the river.