Dryer heat is typically not enough to permanently set dyes these days. Most dyes require chemicals to set. Most likely the dye from the backing fabric came from over-saturation, meaning there were excess dye particles in the fabric. Fabric fibers can absorb only so much dye. When the manufacturer does not thoroughly rinse out excess dye, you get the excess dye in your wash. However, dryer heat is usually not enough to set it so Synthrapol still works.