I've seen this a number of times on my dens. Its funny the only time of the season when I do see this - its appears to be during the high heat months. Seems much worse through out the heat waves.
I do believe its too much water at once coupled by the heat. Perhaps its just going into the canes to fast.
It shouldn't effect the plant. I have had this happen before and also I have had plants where slugs have eaten half of the cane and they continue to grow tall and bloom. Just keep an eye on it to make sure that it doesn't get infected with bacteria. It should thrive nonetheless. Dendrobiums are very tolerant and adaptable. :O)