Russ Allison (not asking me), but searching for the root cause of cotton conflagration, explains:

"Recently, there was a smell of burning from the oven and when 
taken out, the cotton wool was "burnt" and smelled like it.  There 
had been no flames (sorry about the English of that sentence!)
I believe I am right in saying that cotton contains oil.
May that have been the cause and how come, after the many 
hundreds of times we have done it, the wool burned that time?"

Much cotton wool nowadays has never been in a cotton field, but rather is synthetic. Could that be the cause?

Terry L Marshall
Histopathologist
Rotherham General Hospital, Yorkshire