Compressibility causes IAS, and therefore CAS, to be too high. Compressibility is a function of TAS and, for a given CAS, as altitude increases, TAS increases so compressibility increases and the gap between CAS and EAS increases. This means that if you climb at constant CAS the TAS will increase and EAS will decrease because CAS is always higher than EAS. Logically, therefore, if you descend at constant CAS then TAS will decrease and EAS will increase as compressibility error decreases.