Tank bottoms are typically tempered improving the strength of the bottom pane of glass and are sometimes a slightly thicker glass. Its built to support a ton of weight, both water and rock/sand.
This is majorly over thinking whats going on with the glass:
Water pressure on all sides of a tank balances out, the glass bends and supports the water pressure evenly without major stress that leads to cracking. A leaning rock displaces that pressure and creates a point of slightly greater pressure on the pane of glass. If the glass has any imperfections, cracks, ect it causes that pane of glass to have a much greater chance of failing. But the real problem with that leaning rock isn't just that it increases the chances of failure because of an inconsistancy of weight distribution but because there are vribrations in your tank from the pumps, skimmer, powerheads, ect. Even driving cars down the road, you walking next to the tank, ect. The rock acts like a jackhammer on the glass creating vibrations through the glass that gather at those chips causing those chips to create a crack through the tank.
You don't cut glass like you do wood or something, you create a vibration in a chip along the surface of the glass, a little pressure added to that chip and the glass breaks. In theory, a leaning piece of rock helps to create the conditions needed to crack a tank.
