That's common in non-mature clumps of black bamboo (and some other species). If it really bugs you, you can probably get them to grow a little straighter by tying them to more vertical culms or to a stake. When they harden off, they'll be more erect and you can remove the twine.

Probably your "shoots" are whip-shoots, not culms. Whip-shoots are rhizhomess that break the surface, happens often in containers of running bamboo.

Your rhizomes hit the the container, sometimes stay below the surface and run around the container, sometimes break thru the surface at various angles. True culms break the surface intentionally, are often much bigger than whipshoots, and more or less vertically.

If you want only true culms, you'll have to prune the whip-shoots.

"plenty of room" to run could mean 10's of linear feet to a running rhizome, not your comparatively puny container.

unautre-
I agree that sometimes this happens with whip shoots, but I've got plenty of culms on my nigra. my Ph. vivax aureocaulis, and several others that have broken the ground at 30-45 degrees...and they are real culms...1/2-3/4".

Have the culms grown to full height at those angles?

My nigra, vivax, henon had shoots culms that came out let than perpendicular, also in containers, were disturbing, but then as the culms got some height, they straightened up just about vertically.

I just encourage them to grow straight.They do.