What can cause sudden death in 5 week old?

We have an outside brooder/small coop/run that is currently holding 20 chicks, 5 weeks old, different breeds. We moved them out there after 3 weeks, and they've been through some cold nights, with a heat lamp. There have been no issues. Last night it dropped into the low 30's, not the coldest night they've seen, and today I found one dead right under the heat lamp. I inspected it as best I could, and found no injuries. Crop seemed fine. They are on medicated and there is no bloody poop out there. Just seemingly died all of a sudden, laying underneath the heat lamp.

So, obviously you can't diagnose this, but I was wondering what kind of things might cause this. We've had several batches out there, and have never lost one. Do they just die sometimes?

All the others appear to be healthy. I'm at a total loss...