Why is the risk of SIDS greatest between 2-4 months of age?

This is a statistical fact.
90% SUDI (Sudden Unexpected Death in Infancy - SIDS & fatal sleep accidents) occurs in babies less than 6 months of age.
https://rednose.org.au/article/what-does-sudden-unexpected-death-in-infancy-sudi-mean
https://rednose.org.au/article/why-are-safe-sleeping-recommendations-so-important

The risk to babies is greatest in those babies who are placed for sleep in an unsafe position or in an unsafe environment.

https://rednose.org.au/article/what-are-the-risk-factors-for-sudden-unexpected-death-in-infancy-sudi