An employee was pinned against a coal rib by a coal hauler. The IE had walked to the #5 entry to assist another operator establish an alignment marker on a coal rib. As IE was walking back to IE's equipment, hauler (B) was traveling from the #4 entry towards the feeder and contacted the parked hauler (A) located in #5 entry, allowing hauler (A) to slide, pinning the IE to the rib.