I wonder if there was just too much ray tracing going on. I have the clouds masked by a distance shader so no clouds are close the camera, it's than merged with a SSS shape to give a path to the horizon for the Sun to poke through. The two freezes happened when it got to the center of the frame and rendered the sun and blank space between clouds and the ocean below.
Without the mask to the horizon, ocean, and turning down AA a bit, I can render no problem. But usually I can render clouds no problem with max 0.6 AA as long as they aren't multiple V3 intersecting, if they are render times jump 100 fold, but even than, they'll render all night without a crash, giving me a couple tiles finished by morning.