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There was nothing behind the fence Joe. What might have affected it was that it has three rods running vertically through it. Possibly they pulled the wood in an unusual way.
I don't think it negates in any way what you have been saying, it's just going against the grain so to speak. The key phrase in your old text is "tend to warp opposite the annual grains."
An old friend once told me that each piece of wood has it's own "personality" - beware of the Rebels. I don't know a great deal about wood, I was just curious as to if and how the damage could have be prevented and how best to repair it.
I believe there is no shame in failure. Rather, the shame lies in the loss of all the things that might have been, but for the fear of failure.
Kelly, The how not to is WAAYYY easier than the what to do now.. I like your idea of epoxy and the appropriate filler. The 406 is what I use for things like that although a "that" hasn't occurred in many years. I'm not sure the rods are the culprit as I've seen quite a number of signs with those in dandy condition....I go with the idea that the boards should have been altered (grain wise) and they are just too wide to begin with. The least I'd make do with is the riff cut which you (or rather whoever) could have gotten from those same planks easy peasy.
The key phrase in your old text is "tend to warp opposite the annual grains." 
I don't know a great deal about wood, I was just curious as to if and how the damage could have be prevented and how best to repair it.
Denver Chapter of the Letterheads