Most likely a blocked tank vent that's pressurized the system, creating back pressure that's causing the pressure to try to escape wherever it can...which is back through the toilet. Don't use the toilet again or attempt to pumpout until you've cleared it.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning out the thru-hull...use a screwdriver blade, ice pick, whatever works. If that doesn't relieve the pressure, you need to clean out the other end...but before you remove the vent line from the tank, open the deck pumpout cap to relieve any pressure.
You'll find a lot more detail about blocked tank vents--what causes 'em and how to prevent 'em and why it's VERY important TO prevent 'em--in the archives of the plumbing and sanitation forum...search "blocked tank vent"...and also in my book (see link in my signature). And I'm always glad to answer questions.
Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein