1. Incorrect water tank design, some people want reduce heat lose they didn't leave a vent.
2. Incorrect installation, vent is used as other pipe.
Like some people connect the vent as over flow pipe, for saving water the end of over flow pipe was put in a basin or bucket, when water level higher than pipe, it’s equal vent pipe is blocked.
3. Vent is iced in the vent.
Not use hot water when outdoor temperature down to minus.