I've noticed my bambusa multiplex doing a similar thing any time the environment changes. It gets stressed looking anytime I move it to a new area with different conditions. It was highly stressed when I got it, too, while a japonica in the same box showed no stress whatsoever.
Do you have any pictures? I don't get what you mean by saying the leaves go dark green and dry out. mine just dry out from the tips. It appears to happen like you said on only old leaves, all new growth looks good and the whole thing keeps pumping out leaves faster than the older ones dry out.
I don't think bambusa leaves (especially on my multiplex) last as long as leaves on other plants. I've had leaves on japonica that didn't start dying for years, while the multiplex has always been in the process of loosing leaves somewhere.
I don't know what causes it. I'm not sure anything is wrong with the plant at the rate mine is growing, and I also doubt it's salt damage since i've been using distilled water, though it does look like salt damage a bit. I just assumed it was normal since this is the only plant that does this often and it's done it since I got the plant 4 years ago. Does iron do similar things to leaves? I watered mine with a hose for the summer, and last time I filled up a jug using the hose it looked almost like apple juice
On the other hand I put it in drastically different environments every few months so it's always in a mode of adapting to new things.