Proliferative hyperkeratosis is caused by accelerated growth of the cells in the basal layer of the epithelium. This leads to increased cell turnover and consecutive thickening of the stratum corneum. The number of keratinocytes (that later become corneocytes) is increased.
Retention hyperkeratosis is caused by decreased desquamation of the corneocytes. The stratum corneum is thickened because there is less loss of cellular material.