Why are there air bubbles in my tubes of watercolor paint?
Initially, the gushing of the paint is due to the small amount of water that generally is part of the product's formula. This quote from handprint.com explains the reason and effect:

Paints are manufactured with excess water in the vehicle, as this reduces the viscosity of the vehicle and decreases the amount of time (labor) and electrical energy necessary to mill the paint. This water mostly lost through evaporation during milling, but also after milling when the paint is left to sit and age or stabilize. Some pigments or fillers absorb water very slowly, causing them to expand: these are the paints that &quot;explode&quot; or squirt from the tube when it is first opened, because they were not aged adequately before packaging.

The amount of water involved varies from pigment to pigment (some need more grinding or more aging), and brand to brand (due to production techniques).
I would ascribe the air bubbles to an unsophisticated production process, though, in which the tube is not completely filled or improperly sealed.
Oxygen could have formed due to ongoing chemical processes inside the tube, but that should also have been considered before packaging.
As for subsequent - and erratic - eruptions, apart from pressing the tube by hand when opening it, which can be avoided, I think this can only be ascribed to changes in atmospheric conditions: temperature or air pressure, of which the first seems the most likely culprit: a rise in temperature would increase the pressure inside the tube.
To be thorough: I don't suppose the tube was stored somewhere where something could have been (temporarily) put on top of it, or where someone could have 'played' with it? That would make for an obvious cause as well :)
Bottom line: always open tubes above your palette so whatever comes out can be used.