By upending the urn and spilling out its contents you cause a billowing cloud of necromantically infused ash to fill the area with negative energy.
This reminds me of an amusing anecdote. A friend of mine was diving with his wife off the headland of some island in the Aegean, when they came across a load of stoppered clay pots on the seabed. Pulling the stoppers off a few, they were surrounding by billowing clouds of "mud". Later, in the bar, they mentioned the pots to one of the locals. He was like "Oh, please don't go diving there! Everybody around here throws the ashes of their relatives off that headland." Luckily they didn't mention they'd been happily popping all the stoppers off.