its a bit like pressure i think, where if you decrease pressure the eq shifts to the reaction that creates more moles of gaseous products. However since these are liquids, we are talking about concentration, but the concept is the same, so if you decrease the concentration, then the equilibrium will still shift to the reaction that creates more moles of liquid molecules.
As for this. "I thought that equilibrium only changes when you change the conc of only one substance." these changes affect the system overall, not just individual substances, so the eq does shift.