What causes a car battery to cross polarize?

A typical car battery is made out of Lead, Lead Oxide and an acidic solution of water and sulphuric acid.

When the battery is used, discharging and recharging, there will be particles formed that due to gravity will drop down and form an increasing layer of conducting material at the bottom.

With use, the cells will eventually shortcut due to this layer building up and "touching" the plates of lead/lead oxide.