What causes a toilet to flush on its own?

A slow leak - when the float gets to a certain level due to the leak the toilet flushes.

It cost me $65 to have a plumber tell me that.

Check the tank water level, it should be at the mark in the tank or 3/8 to 1/2" below the top of the over flow tube. Now look at the flapper, If it has wavy edges on the flat surface, replace it. The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the ballcock and make sure it's connect to and draining into the white overflow tube. If the leak is at the base of the tank there are two brass bolts on both sides of the flapper, if the tank is loose, leaking or wobbly, snug up the bolts. If there is dampness at the base of the bowl, pop off the china caps and snug down the closet bolts. I would check all this out before paying 50$ for a service call and 50$ a hour thereafter. More questions? I'm as close as a click. TOM

- And I wasn't smart enough to post the question and ask Tom!

A word to the wise.

Hi Judy, Glad to see you.
Hope your holiday was the greatest. Cheers, Tom