Is this an outdoor pen? That raises two possibilities: biological and chemical.
if ladybugs can get in, so can all kinds of microscopic pathogens, which may have taken a while to build up to dangerous levels, then seemed to hit all at once.
The other possibility is insecticides/weed killers, etc. getting blown in.
The only way to really know would be to get a biopsy done on one of the dead animals. Probably a bit late for that though.
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0.1 1987 Coastal Carpet (Boots)
0.1 2006 Western Hognose (Bebe)
0.1 age unknown Cane Toad (Hengo)
0.1 2005 White-Banded Sheen Skink (Minerva)
1.0 2006 Northern Diamondback (Queequeg)
1.0 2006 Madagascan Speckled (Sigmund)
1.0 2008 Bull (Winkle)