Gentlemen,

It's hydrogen gas bubbles. They are produced in the cell. One electrode produces Cl which mostly dissolves as hypochlorite/hypochlorous acid and the other electrode evolves H2 gas which is not as soluable in water

Definitive ChemGeek thread is here

The pH rise is both due to aeration/CO2 out gassing and the production of hypochlorite which is alkaline.