Constant flexing of elastomers, including polyurethane, creates heat build up within the material.  When a stress is applied to the material, there is small time lag before the material takes up the corresponding strain. The difference in the stress-strain curves from application to recovery result in energy loss (HYSTERESIS), which converts to heat.  Polyurethane does not easily dissipate the heat generated and if allowed to rise too far, the built up heat can revert the urethane to a thermoplastic state, resulting in a blow out of material and in the case of wheels , a debond from the substrate.  Changes in design, stoichiometry, choice of curative and /or prepolymer can help prevent failure.