Why is 316 Stainless Steel rusting?

A. Can your furnish photos, including close-ups of the corroded fissures?
Does rolling refer to the sheet stock prior to welding into tubular shape, or was some mechanical forming performed on the tubes?
Any specification on the tubing, e.g., ASTM A268, ASTM A511, ASTM A554...?
How was the tubing cut to length, and were the ends deburred and cleaned thereafter? Passivated?
Were ordinary steel tools used on the 316, or is there any contact with other metals?

My guess: The fissures suggest cold-worked tubing that is stress corrosion cracking (SCC). Possibly, ASTM A554 tubing (ASTM A268 & ASTM A511 require annealing) or some non-grade stuff. A true fix might require annealing + passivation, or may not even be possible if the surface is heavily cracked. If a large amount of material is involved, consider having a metallurgist or testing lab check whether it meets specifications. Very light rainfalls carry surface contaminants to the bottom where they concentrate as the water evaporates. There may also be some initial contamination from forming or cutting. The contaminants attack the heavily stressed, possibly microcracked material. If the material is installed architecturally, wash, apply an automotive chrome cleaner/polish and then wax.