That sounds very strange. As a general rule, there's a separate soap solenoid for each individual bay, so for all bays to suddenly be getting soap on the wax selection all the solenoids would have to fail open at the same time. Even if that's what caused your problem, closing the soap valve for one bay should just shut off the flow of soap from the tank and not affect the operation of the pump at all.
My first guess for the cause would be a leak between the soap and wax tanks. On older D/H equipment the divider between the soap and wax tanks is just welded in and sometimes leaks. If you can determine that's the case, you can drain, clean and seal the seam with good quality silicone sealant. I've done it a number of times.