For structural engineering purposes, a 16D Common nail is rated for 141 lbs of pullout strength in nailing two 2xs together, and 165lbs in nailing 1/2" sheathing to 2xs, IIRC.
The wood looks OK in those photos, so I wouldn't suspect rotting or water. Vibrations or something else must have loosened them over time, causing them to fail. Either way, nails are the WRONG thing to use in that application, no question about it!
