Vessel flushing. The blood pushed out of the vessels by the pressure of sitting and a relative hypoxia occurs. When the pressure is relieve the hypoxia dilates the vessels causing a flush of blood into the vessel making a red mark.
Series of events:CO2. It is a series of biological processes. Initially the pressure compresses blood vessels & occludes them, blocking the circulation to area & depriving the local cells of oxygen. When the pressure is relieved, the "suffocated" or ischemic cells in the area build up co2 and other humoral factors which make the local blood vessels dilate and open up (causing the red appearance) to deliver oxygen!