Thermal risk is where an area of glass is heated to a temperature differential beyond the capacity of the glass to take the stress created. It is not the absolute temperature but the difference between one area and another.
When thick glass is used or
solar control glass.
Shadows on the glass, orientation, heaters, the type of glass, angle of glazing, location, internal walls or blinds and the efficiency of the glazing system, can all effect the relative temperatures across the glass.