That's a very badly written story - it's not clear whether the birds are eating gravel or stealing gravestones. Had to search for the Strabane Chronicle to find the details.
Seems the crows are stealing blue, egg-shaped, decorative gravel pebbles.
Possible reasons? Crows like eggs. Crows like shiny things. Crows are intelligent. Crows can be trained.
Perhaps a sneaky garden centre owner has trained the crows to steal the stones which he then resells to the bereaved?
There's a true story on the Snopes Urban Legends website about crows/starlings stealing money from a carwash.
http://www.snopes.com/photos/animals/carwash.asp