One article had this to say:

"Earlier this month, Busch radioed his crew near the end of a NASCAR Cup Series race in Watkins Glen, New York, and asked a doctor to give him a “shot” after he finished, The Associated Press reported. Busch had been struggling with a sinus cold that was worsened by the intense G-forces and elevation changes at the road course on May 10, according to the TV broadcast of the race."

It sounds like it developed with a sinus cold, and he probably tried to continue with racing instead of taking time off to get recovered from the sinus cold.... and it developed from there.   Very understanding to see an athlete take that position: "I can push thru this."

The other thought that I had..... because Kyle Busch was so well known, it probably brings a lot of attention to the risk of sepsis.  If he had been a "No One", there would have been no news on it. He would have become just another person would die from sepsis - a statistic, so to speak.  His death is drawing attention to the risks of sepsis.