My guess
:
The wooden shell of the log always tends to expand and contract a tiny bit even when not in use, depending on its own rest moisture content, current room temperature, moisture content of the air and the specific wood used. With the constantly applied heat then, the wood dries out even further over time and so overall tends to shrink more, than it is expanding... until it finally meets an unmovable object in its way before all the shrinking is done for good: the heat sink.
Seems to work out quite fine for most (or at least the greater part) of sold units though, from what I read here; i.e. some seem to crack, but most don't. So might well be, that if all the wrong conditions materialise at the right time... crack might go the shell, if you're really unlucky!
The side with the cut out for the power socket is probably also the weakest spot of the shell and the power socket itself is quite big and also won't give. Stress from an inflexible cord might also play into it, but I think unlikely to be the sole or even main culprit here, as the unit is much too lightweight to resist the pull of the cord much, me thinks.
Again: just guessing here