My guess is they thought they were not live.
My story goes like this. A co worker unloaded cinders from his fire place from a week pryor. Placed them in a plastic bucket on the harth for the morning. In the morning he placed the bucket in his branb new thuck by the tail gate. He had a spray in liner and while driving at hiway speed blew cinders up by the cab and started a fire sice the sray in liners are flammable. The truck was completly consumed in 5 min once he stopped.
Moral is that cinders can remain active for as much as weeks in the right conditions.
Also. Merry Christmas!