[MBAtarga]

All the search results I see mention the same suggestions above - a clog, a vent issue, or a crack in the toilet.

I also learned something - "Contrary to popular belief, the plunger is NOT meant to force a clog down the drain. Plungers are meant to be used to create suction in the toilet trap, causing the blockage to break up and then flow down the drain when the plunger is pulled out of the bowl. Pushing hard on a plunger usually just blows out the wax seal between the toilet and the flange. Then the toilet has to be pulled and resealed."