Just curious as to what the requirements are for a public league to close .. 0 owners? <15% ownership?

i would imagine it would have to be 0 owners in the league or the league is so buggy that JDB has to kill it.

Nothing happens automatically, but if a league is stuck because the ownership is too low I can always kill it. That's not likely with a public league, but could be with a private league. If there are significant issues and the league has a general consensus that it's totally screwed up, you can always petition to have it closed, probably via a poll (which will be coming to the general leagues with the next website update in a few weeks)