Hotpoint ice diamond ?

I have in garage old Hotpoint Iced diamond tall freezer (circa 15yrs) and 'er indoors' kept putting stuff( empty boxes with jars in etc on top of it). Top now clear. Still works fine except outside on top it freezes in patches. When first spotted had to use hair dryer to release boxes etc.
Told er not to put stuff on top, but then thought"I'm talking to a woman". Go in garage three days later box on top. So clear top or not these patches of ice appear. What could be the cause?

Many thanks in advance