A recurring theme - water causes problems if it gets to places it is not supposed to be in.
This chimney is a bit weathered and has no protection from the rain. Exposure to water over the years has deteriorated the mortar between the bricks. It's now at the point where the bricks are loose and have the potential to tumble off the roof. Not good if you happen to be standing in the back yard.
This is a non working and abandoned chimney, when the roof is replaced it would be a good idea to remove the chimney to below the sheathing and roof over the opening.
In the meantime, the chimney needs a rain guard / rain cap to be installed once the loose bricks are removed and a stable base is established. This will reduce the chances of water intrusion and help preserve the remaining mortar. No more bricks falling off the roof.
Brian Jovag, owner of Jovag Home Inspection.