Maybe they kept dodging into magma during fights?
Again and again Onul checker barracks for some hidden danger that could cause injures during training session - nothing! It was just a room with armor stand in it. And with one wooden training spear hidden under each floor plate - so called "danger room". But after first death he forbade activating them - that was the first reason he thought of. But deaths continued...
Sieriously. Give us a better hint.
OOC: Actually, I am caged by Urist's and Mormota's questions. I'm doing my best maneuvering in this tight box Onul spent whole day thinking, thinking and thinking. His head was buzzing with thoughts. The clue was so close, he knew that!
Again, he tried to summarize:
1. It wasn't forgotten beast's or titan's or demon's syndrome.
2. It wasn't injure caused by fighting or sparring.
3. Accoding to medic's word, symptoms looked like death caused by over-heating. But there were no magma or fire near place of their deaths.
4. It wasn't any of cavern beasts' attack. Or dragon's fire breath.
5. Each of dead recruits choose an iron spear as his weapon, and mostly adamantine armor (but there were bronze and copper pieces, for the first time). Sure it's because of variety in available armor pieces due to hard work of legendary armorsmith.
6. No deaths were among civillians, including heirs of dead recruits, so !!sock!! isn't a case.
7. No ghosts were here.
...
Onul walked down the spiral ramp to third cavern and magma pool opened there, just to spit at magma and watch steam and smoke arise. That could stimulate his imagination.