A common cause of plywood delamination is water and moisture. Plywood comes in different moisture ratings, depending on the composition of the glue used to hold the veneers together. If you use an interior-grade plywood for an exterior application, the glue will be dissolved by rain and moisture, and the plies will eventually separate. To demonstrate ply separation due to moisture, do this experiment: Take two, 3-inch square pieces of plywood, one interior-grade and one marine-grade. Place them in a bowl of water and come back in about two weeks. You will see the water has seeped into the interior-grade plywood, and delamination occurred. The marine-grade plywood should be unaffected.
Quality control may be lax at the manufacturing plant, and the glue may have gone bad. The main reasons for glue going bad are age and freezing temperatures. Many glues are water based. If the glue freezes, the water turns to ice, and separation from the chemicals occur. Rather than throw the entire batch of glue away (which can be hundreds of gallons), an unethical operator may use the glue anyway, hoping all will be well.
An unethical plywood manufacturer may try to skimp on costs, by using as little glue as possible. When the plywood is fresh, the glue may be adequate. As the wood dries out with age, the glue also dries out and reaches a point that the stresses overcome the glue, and the plies separate.
A combination effect may take place. First, the plywood was improperly installed, such as using interior-grade instead of exterior-grade plywood. Second, insufficient or bad glue was used in the plywood itself. Combining these two forces result in delamination.
A study conducted by the Forest Products Journal in 2003 concluded that nailing plywood on its edge causes micro delamination. When you drive a nail into the edge of a plywood panel, the end effect is it wedges the wood apart. This allows moisture to seep in. Over time, the moisture dissolves the glue from the inside out, causing delamination.