tl;dr at the bottom.
An aging, crippled veteran(Ability to stand lost) gets possessed and drags himself out of the hospital. Slow going, because he's crawling around on his belly. He claims the adamantine extraction workshop in the basement and heads down into the vein, looking for only Armok knows what.
He crawls to the egress level, where there's a hallway and airlock to the caverns, and just sits there. Whether he's slowed down due to exhaustion, or dehydration, or pathing issues is unknown. The point is, he crawls down there and is forgotten for about a season.
Out of boredom, I'd stationed the militia down there a few seasons ago and sent them out to slaughter crundles and elk birds. Dwarves being dwarves, and the equipment system being what it is, the end of the hall where they had been stationed is strewn with abandoned clothes and armor and half-empty barrels of booze. No one had been down there in some time. Until now.
Urist McThirsty senses unclaimed booze in the egress tunnel. Instead of going to the stockpile on the farm level, the stockpile in the tomb that's supposed to be for the workers clearing the stone there, or even the main stockpile on the top floor, he heads down the the same level Urist McPossessed is crawling around in. He steps into the egress hallway, totally ignoring the dwarf scribbling lovecraftian glyphs in the stone and babbling in eldritch tongues. At the entrance to the airlock, he finds his prize: A barrel of whip wine.
Being a dwarf, he drinks it. He spends a few ingame days down there, just drinking that whip wine.
Meanwhile, McPossessed comes to his senses long enough to notice this dwarf walking by him and ignoring and his need for materials. A vein throbs in his head. He snaps. They all need to die. All of them. Sure enough, his trusty ≡adamantine short sword≡ is where it has always been. He draws it, and the entire world turns red.
McThirsty, having sated his need for alcohol, decides to be a responsible dwarf and bring the barrel back to one of the more accessible stockpiles. He begins to head back upstairs
Urist McThirsty cancels store item in stockpile: Interrupted by Urist McCrippledAndBerserking
Epilogue: McThirsty was slain after being cornered by McCripple. McCripple, unable or unwilling to go back up the vein, is promptly oneshotted by the current militia and his body thrown committed to the magma. His equipment has yet to be retrieved.
tl;dr: A dwarf walked by a crippled, possessed, ex-military champion dwarf to get a drink from the end of an empty, abondoned hallway. When he went back by him, McCripple had gone berserk, and gutted him.