It depends what you mean by shut off POPPING OUT? More info needed
Check the flapper and/or the ball cock assembly. Either or both probably need to be replaced.
Where is it leaking ? Flapper in the tank, replace flapper. Between tank and bowl, separate and replace the seal. Toilet to floor. Replace the wax ring under the toilet, making sure it is thick enough to form a seal and the bolts are installed correctly and holding the toilet down.
Shut the water supply off to the toilet. Flush the toilet. Pull the old flapper out and replace it with the same type. Turn the water back on and test the toilet.
Check the tank and make sure it's full. Check the flapper and make sure it moves enough to drain the tank into the bowl.
I would suspect you may have a leak in the toilet. You will need to remove and replace it. Of course water will evaporate over time if the toilet is not used occasionally. or The flapper inside the back of the toilet holding tank may not be sealing correctly. Sometimes just adjusting the flapper may work (it may be caught on the chain)...other times a new flapper will have to be purchased (a very common item at any hardware store) and installed. The part should be around $5.00.
I think the flapper is not closing properly. Try adjusting the flapper or replacing the flapper with a new one.
5 years
Wrong adjustment
remove and install a new filler valve and get a proper flapper for your toilet at a plumbing supply
the flapper in tank is not sealing after flush.change flapper.
Replace /bulb / flapper /disc and check the over flow tubeA:The kits are inexpensive and come with the float and the flapper assembly.
My behind