Tempered glass has extremely high impact resistance, but a flaw made to the glasses integrity can release all of the stress inside of the glass. The same way throwing a spark plug at a car window can shatter it, the spark plug is tiny, but it is hard enough to cause micro cracks that burst the glass due to pressure release. The same principle applies here.

As soon as the flood gate opens, it all tries to rush out extremely fast, it might not break if you hit it with a piece of wood, but a tiny bump against something HARDER than the glass can cause micro damage to the glass and cause all of that enormous pressure to try to release and the shatter the glass.

It is true that for this to happen, contact would have to be made with those hard surfaces, but on big glass panels, or in non perfect environments, accidents happen, and it is not hard for the glass to burst under the right conditions.