Peter it was a combination of maintenance issues, pour building practice.
The ash pit door is at ground level. Not for that climate.
The shoulders are rotten.
Lets play the home inspector detective.
First I will make 2 comments.
#1 The chimney was an after thought? OR #2 They did not install in properly. It should have been masonry course bonded into the wall if there is one. If not it should have been anchored into the structure.
That would stop the chimney from pulling away from the structure. I suspect after thought. Many reasons why.
#1 Lot slope and gravity. Is the lot reversed sloped? That would be my first observation for an hypotheses.
Look at the ash pit. Should be in the basement if there is one.
Way to high. Condensation = Water will freeze inside when the chimney cools and creates RH from the exterior tempt change. It a dam door.
#2 The shoulders are rotten. Water will enter the cavity between the masonry and the wall that is behind the chimney.
I see they painted the brick. Its a no no.
After thought chimney I suspect.
I could go on but why.
SUSPECT: defective chimney. shoulders are defective in application and build.
Defects: Shoulders, masonry bed bond lateral and serpentine cracking above the ash pit clean out the width of the chimney.
The chimney is leaning xx degrees away from the homes main structure.
The brick masonry does not seem to be tied into the structure with anchorage. There is a gap that runs laterally in height of the chimney gaining in separation from the base being the smallest separation to the peak or the gable being the widest in separation at xxx inches…yada …yada yada,
RECOMMEND: A licensed mason evaluate if the chimney can be salvaged.
If it for a furnace it can be strapped to the unit and a liner used.
This may be danger and health hazard to the home owners and also allow water and weather to enter into the structure.
All the best…