I have seen that. I have no idea what causes it. The dead
leaf actually looks like a separate issue from the purple/bronze
edges in the second picture (see how the stem is rotted on
the dead leaf).
There are a few nutrient deficiencies that can cause "marginal
necrosis" (
http://4e.plantphys.net/article.php?ch=5&id=289 ),
but the whole plant in the background does not really remind
me of any of them.