By the widening base, it looks like a fireplace.
I have seen chimneys crack like that due to severe creosote based chimney fires but the cracks appear farther up the actual chimney not behind the fireplace (if it is a fireplace).
Wonder if something venting into the chimney was overfired and the temps rose quickly, causing thermal shock? This is the phenomenon that causes cracking in chimney fires. The flue gases were chugging along at 300-500 or so degrees F, then the chimney fire starts and temps rise within 2-3-4 minutes to 1200+ degrees…that’s a shock to the chimney as the tiles expand much quicker than the unit masonry does…pressure and the crack occurs.