Why paint cracked?

[Howie]

I canvassed & painted a canoe for a fella last year, and he was using it last year with no problems. This year he tells me he put it in the water & paddled while the water was just above freezing. When he took it out of the water and tipped it over he noticed cracks in the paint around the keel (see pics). He's certain the canoe didn't get dinged while in storage. He has since sanded the area down and says the cracks do not extend into the canvas and is in the process of repainting the area. The paint and filler is Kirby's.
Any thoughts? I do notice what looks like a roundish 'ding' like it hit something in the water, but I see no scratch marks at all. Could the cold water have caused this?
If so, why did it happen in just this one area?

 

[Howie]

New info. He inspected the canoe before it went into the water & it was ok. He saw the cracks when it left the water.

My guess is that the canoe might well have been a lot warmer than the freezing water. And when the canvas hit the water it contracted and caused the paint to crack. If so, it's odd that it was in just one area... But that's my theory.