A partly clogged drain line
Partial main drain line clog or full septic tank BackThis animation is of a partly clogged main drain line, or a full septic tank.
Note how the main line fills up, the lower level toilet bubbles and the water starts to back up in to the tub, the fixtures on the upper level are not affected by this clog because the water is backing up in to the tub, if a person was to keep using the bathroom upstairs, the toilet will over-flow and the tub will fill with water, the sinks on the lower level will also drain into the tub.
You will also see that after some time with out using any water the drain line slowly empties, this gives you a few flushes and maybe a shower, depending on how your home was plumbed.
Keep in mind water always seeps it's own level, if it can not go down the drain it will back up in the lowest fixtures, and will overflow, in the fixture lowest to the floor, if you have a shower stall on the lower level, that will over flow before the toilet, If you have a bad wax seal at the base of the toilet, the water will run out from the bottom of the toilet.
A heavy duty main sewer cleaning machine will be needed to clean this clog, sewer machines are very dangerous you should call a professional.