Relics/ dhathu have supernatural powers?
The supernormal acts can happen in 2 ways:


The Person has Jhanic powers and made a strong determination that so and so should happen with the relicts
A deva or being which the power influence matter in the human realm influences the relicts


Generally in my opinion the latter is happens when relicts mysteriously appear in shrine rooms and places of worship. If the relicts do not get veneration in where they are a deva may bring them to another place where they might get the veneration.

The former case may be associated with crystallizing of the relicts. E.g.: http://www.bdcu.org.au/bddronline/bddr12no7/Mun168.htm