They are know as pot breakers. Very common with S. masoniana. I think they are related to the sweet potato. At least that's what their roots look like.

How long did it take it to do that?

Mine grow underground and push for maybe a year or so before it pushes that bulge shape. They do have large rhizomes. So the pot just seems to have happened overnight but the roots grew for years to do that. (At least for me that is.)

I potted this guy up in april or may...3 months? to bust the pot.

What secret soil formula do you use?