The toilet bowl can only be filled so high (unless it is clogged) before it self-levels at a specific depth. Now, this can take a minute or two after the fill stops. After that time, for it to chance, there has to be some other force, or a leak. A back-to-back toilet that isn't plumbed properly can cause water to affect the toilet opposite it, as can high winds creating pressure waves in the vent system (this can rock the water, causing some to slosh over the weir and lower the level). You can determine the max level by slowly pouring water into the bowl (pouring too fast can make it flush), then letting it sit for a few minutes...that's as high as it ever will sit.