Apparently due to large convection currents within the star.
The sun has them too but smaller and confined nearer to the surface.
Betelgeuse’s size affects their size they are much larger,
the currents can reach further down into the interior of the star and in turn,
make the star bubble out at its surface to a much greater distance.
The section of the video I saw explaining it is cued at this link..
https://youtu.be/BDB4ZF8jX9Q?t=10m27s