There are many examples in the story proving Min to have a short temper here are just a couple of those reasons.
One of the quotes proving the easily irritable potters temper is when Tree-ear is watching him work he exclaims "How dare you come here. How dare you touch my work".
This quote supports the old potters habit of being snappish and volatile because Tree-ear did not do anything incorrect or anything to upset the potter. The only thing close to wrong was Tree-ear watching him work. Also he is working outside were any one can see him.
The next quote that proves the quick temper of the potter min is " tree- ear counted the piles of shards there were 5 piles