Well, I can't say 1st hand because my small plants do something similar. I think what you're describing are sometimes called "whipshoots" which usually occur later in the season. Early spring is usually when the true, upright culms will shoot up, but I've noticed these will droop too when they get heavy with leaves. I think the problem stems from the "grove" being too immature to produce big, upright culms. Someone please correct me if I'm wrong.

I agree with Thuja in that many of my young Phyllostachys tend to lean. But, one of the exceptions here has been Nuda. It has really been very upright from the getgo and mature Nuda groves are very upright.

Another possibility to consider, is that I've learned a lesson in raised bed plantings. Older culms in raised beds where I mixed my own soil continue to lean to an extreme because I do not have enough clay in them. In fact, I used only fine materials and no real soil in the beds. Now I am trying to repair all of them but it is hard to add soil to an existing bed full of culms. Don't know the growing conditions of your Nuda but thought this may be worth mentioning.

Uh oh. I built this bed by piling everything I could find (leaves, twigs, sod, grass clippings, compost, bark chips), and it's like a berm. It's a pretty loose, sandy mix with lots of air space and good drainage. Imagine bamboo planted on top of a well settled 20'x20'compost pile. I thought this would be terrific. Guess I have a problem.

Sounds similar to my situation - the bamboo is quite vigorous due to the excellent growing medium but if you step in the bed the 'ground' is quite soft. Mine is horse manure/sawdust mix plus leaves, bark mulch, some bags of top soil and some bags of cow manure. It breaks down quickly and I have been adding more horse manure/sawdust mix to keep filling the beds. I think that the anchor roots just can't quite get a firm grip in it, not firm enough to support 15 foot culms so I've pruned them all down to keep them upright. Now I'm tossing 6-8 inches of composted cow manure mixed with sand on top of the beds to try to firm up the toplayer so that the culms can't lean as much. Don't know if this will do it, but the stuff is the texture of wet cement and what the heck, it's manure & silica that the bamboo will enjoy. I may always have a Vivax, Bory, and Atrovaginata hedge.