卩 on the right is the indexing radical, a stylised side view of a person kneeling. It places 卸 in the family of 印, 即, and 卷 - words that turn on bending, settling, or laying down, exactly what unloading does.
午 at the upper-left is a contracted form (3 strokes here vs 4 standalone). It originally pictured a pestle but here stands in for the horse-time portion of an older graph showing harness being lifted off an animal at midday.
止 below depicts a foot and contributes the sense of halting and standing still. Together with 午 above, it suggests a rider dismounting and stopping the journey, the motion that 卸 names when applied to cargo or duties.
