Why is the cation exchanger used before an anion exchanger in a DM water treatment plant?

water has calcium and magnesium ions. If the anion unit is placed ahead of cation unit, the exchange of chlorides, bicarbonates etc will generate corresponding hydroxides, since the anion resin is in Hydroxide form. This will result in calcium and magnesium hydroxides to precipitate in the anion resin unit and choke the resin bed.

Technically, if you have only soft water to treat, you could place anion unit ahead of action unit in a DM Plant.