Is this normally said in German? Is this a cultural thing I'm not aware of?
Or maybe cups are falling from the window of the second flour! And a man shouts "Tassen!" to inform children below!
I don't think it's an idiomatic expression or anything of the sort, just something that might occur in everyday conversation. For example: "Should I bring cups or glasses for the punch?" "Cups!"
Perhaps someone is yelling for cups as a doctor would yell, "Scalpel!"