Is this a bun product, molded skinless, or integral skin product? We've dealt with a number of similar issues in various products over the years. Do you have any fabric or upholstery cover over the PU? What is the environment? Are there high cyclical load/unloads or surface shear loads?
In bun or slabstock, our issue was too light of density, it was just too "wimpy" of a material. Went to higher density and problem solved. In the integral skin process, there are a multitude of factors which will degrade the properties. Moisture, poly and iso out of proportion, mold temp (affecting skin thickness), old material (improper cure), etc etc etc.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.