The sinking bubbles in Guinness and stouts in general have intrigued drinkers and scientists alike.  Research into this phenomena has been highlighted before on this blog (post linked here).  The latest research published in arxiv (academic paper linked here) demonstrates the importance of the shape of the glass in generating the fluid circulation necessary for the creation of the sinking bubbles.  Numerical simulations and experimentation show that the standard pint glass which has a narrower base cause the falling bubble effect whereas a glass that has a larger base does not i.e. results in rising bubbles.  Essentially the small (nitrogen, as opposed to carbon dioxide used in most beers) bubbles are being carried along by the local fluid motion in the glass and the shape of the glass influences the circulation of the beer as it is poured determining a sinking or rising bubble effect.  More explanation is provided in a BBC news article here.