Toilet valve flushing by itself
Small rocks and scale can cause the fill valve to leak allowing more water in. Some can be fixed by turning the water off, then removing the top of the fill valve and slightly turning the water on Put a cup over the top so when it turns on you don't soak your bathroom. Push down on the float let the water run for a few seconds. turn water off and reassemble the valve. wait and see if that did the trick. Usually there will be rust and sediment in the bottom of the tank if it blows out a lot of debris. I like to clean this out with a wet vac so it wont leave rust stains in the bowl. The valves are inexpensive and not hard for a DIY person to change at worst case.