We see the same thing here where our equivalent of Romex has been used as an extension cord. The solid conductors just aren't intended to take the frequent flexing and after a while they fracture at the points which have seen the most movement.
somewhere here on ECN there is a thread where someone used a coiled-up box of romex and it melted down
You could do that with flexible as well if you pull enough current through the coil that the heat can't dissipate properly.
