You might want to pose this question on the chemistry forum as the experts there should be more knowledgeable. I have had courses in chemistry and studied chemical equilibria and thermodynamics but I am not sure why the reaction shifts to the left when the volume is increased. A chemical equilibrium is a dynamic state with molecules constantly combining and breaking up. The equilibrium is defined for constant temperature and pressure, so that if you change one of these the equilibrium will shift; but as you said, NH4Cl is a solid and not affected by change in P.
I am guessing that the shift has something to do with deltaH, the change in enthalpy, which is negative. H is a hard concept to understand without a lot more math and chemistry than you probably have and I don't understand it well enough to explain it in simpler terms. The one thing I do know about it is that the value of deltaH is negative when a reaction gives off heat--known as exothermic: https://en.wikipedia.org/wiki/Enthalpy#Heat_of_reaction
In the reaction, HCl and NH3 are both gases and if the volume of the reaction chamber is increased while the amount of gas stays constant, then the P has to decrease. The reaction of HCl and NH3 depends on their concentration and if the volume expands then their relative concentrations have to fall and maybe this is why the reaction shifts to the left. I am giving you an educated guess, but it would be better to pose your question to a chemistry teacher.