Good morning, Mick
I can theorize on a couple of explanations. The first as you mentioned is trapped moisture. Leather will shrink somewhat over time. The leather discs and the spacers are glued together, so the contraction is typically going to create a gap at the hilt and / or the butt. That's why Bo constructed most his leather handled knives with a tang nut that could be tightened. The other is a chemical reaction (slow oxidation) resulting from chemicals in the fiber spacer (actually "gasket" material). However, not all spacers may contain the same chemicals. The shop has always had several sources for this material and the manufacturing process from different vendors may not be the same. If this latter possibility is the reason, it is apparant that this chemical does not effect brass or nickel silver. There also may be another explanation we haven't thought of.
Best,
Ron