There are a 1000s things it could be but I always look to the most obvious.

What are the temps as of today? High low surface and ambient?

There are no natural chemicals in woods and or organics that have concentrations that kill fast. There is things that are not particularly good but it is more likely to cause illness. Even raw cedar is only a respiratory irritant it is not good but could that kill in a short time? Unlikely.

If it is not simple environmental (probe being pulled loose) I would look to external chemical PAM or NIX being the common culprits. Other insecticides follow. Strong cleaners may also be possible.