The decoupled layer quasi-two-dimensional glass phase of c-axis vortices in optimally doped Bi2Sr2CaCu2O8+delta (BSCCO) exhibits low-temperature metastability, the onset temperature of which is coincident with the peak in depinning current for zero-field-cooled sample preparation. We present experiments in strongly underdoped BSCCO showing similar metastability and develop a model for pinning dynamics where metastability results from progressive loss of thermal contact. The model accounts well for the field-temperature locus of the metastability and for the irreversibility and memory effects. We conclude that the metastability arises from a nonsingular dynamic crossover and does not mark a phase transition.