Back to back toilets can do that if the fitting in the wall isn't the correct one. Older toilets had a slower flush; newer ones must flush much faster to get things out with the limited water. So, if the proper fitting isn't in the wall, some of it carries over to the opposing toilet, rocking things, and since the toilet is full to the weir (where it drains), rocking it causes some water to be drained out over that edge. Voila...the static level is now lower.
Have someone watch the opposing toilet while you flush, if there's significant movement in it, you've found the problem. Now, fixing it is harder as the proper fitting is usually bigger and harder to fit in the wall on a back-to-back installation.
Really high winds can rock the water spot, too, and cause a similar effect, but that would unlikely happen all that often unless you lived on top of Mt Washington!