The zinc is an anode in the battery cell. It is emersed in water- the electrolyte. The pressence of a voltage difference actoss the cell drives a current. Off gassing iccurs at both the annode and cathodes of the cell. One stream will be Hydrogen the other oxygen. The more vigorous the bubbling the more current that is flowing. Thats how it works. Now as to why the potential difference could be everything from potential due to different metals in close proximity to external stray voltage due to improper wiring etc. Good luck