Sinking separation wall

[hakkafusion]

New to the forums, had a question regarding a property I manage.
It's a old semi-detached home, the interior separation wall of the two units seem to be caving in, from the outside, it looks like the separation wall is kind of sinking; however the side walls of the house seems intact.

Anyone able to offer some insight as to what might be the cause? water damage? bad roof?
The house has no basement/only small crawl space, I suppose it's possible that the bottom is rotting?

Advance thanks for any inputs!

Attached is pic of the house, outlined in red and blue, it's not very obvious from the photo/by just looking at it, but the sag/sink definitely there. fyi exterior wall is vinyl/asphalt plating, roof looks recently redone (2-4yrs old)

[pushkins]

Photo's from the inside would be ideal, especially of the wall in question.
That lower entrance roof is attached to the exterior of the home, with it's obvious sag in the middle I think it's issue is a separate one to the wall your talking about.
From the exterior picture there are no obvious signs of a structural failure, there would be no roof load on any interior wall (ceiling load yes) as those loads are on the exterior walls.
Check the floors, walls and ceiling directly below the wall in question, if it's a structural issue with floor joists in the crawlspace then this would show up on all floors above.
Interior pictures are what is needed.

[HayZee518]

that sinking feeling

is the door casing plumb the way its installed? does the door rub on the casing when it's closing or opening?
the way I'm thinking is that the plate[s] join in the middle and are resting on [piers?] or a sinking block foundation.
is there a poured footing running the length of the front facade?
what about the ground makeup at or below grade - sandy, loose gravel?

[BigDon]

Hard to diagnose with just those pics. You say "crawlspace" so I'm guessing it's a pier & beam home? Does it have a continuous footing foundation? Have you checked the piers/blocks to see if there is any noticeable damage to them? I'm gonna guess your problem is the foundation. Start from there and work your way up until you find the culprit.