Stress or unfavorable growing conditions: for example, these plants get stretchy and thin when not getting enough light. These plants like a lot of light. They take pretty much all day full sun here in Mesa, AZ.
This is an older picture of one of my Pilosocereus plants (probably not quite the same species), you can see that right below its branching spot it has a much thinner and narrower section. I got this plant as a cutting and I had it in the shade while I was waiting for it to root. It grew that thin not quite right looking section during the rooting process. The branching happened for me after the growth tip froze one winter, since then I cover it for the few days each year we might have below freezing temperatures at night. Being out in the full sun brings out the best of its coloring.
