The upper layer of the cargo remain cool due to evaporation. The lower layers gradually warm by heat transfer.
Rollover may occur withe lighter more bouyant lower layer rapidly moving to the top replacing the heavier cooler layer whcih falls to the bottom. This warm layer is now exposed to lower pressure and rapid boiling occurs which can lead to over pressurisation and extensive damage. This a very serious occurrence.
When loading mixed LPG cargo the warmer cargo should be loaded first which reduces the amount of evaporation.