Wooden decks on the corner units?
—”Wooden decks on the corner units?”
Standard stuff, masonry exterior, and mostly frame interior.
My guess is a long-term small roof leak and the water frost wedging, 10% expansion at freezing, inside the wall with the freeze cycles each cycle, the crack is slightly larger.
Could also be a plumbing leak or digging near/under the foundation.
Always water problems.