The abstraction of a hydrogen atom

The H-CN bond is stronger than the O-H bond, the abstraction of a hydrogen atom from HCN by the tert-butoxy radical is endothermic by 528 - 438 = 90 kJ mol-1. (These numbers are from Table 5.3 with the bond energy of CH3O-H as an approximation for that of (CH₃)₃CO-H.) The first initiation step, formation of the tert-butyloxy radical from a peroxide, is also endothermic.  Because both initiation steps are highly endothermic, the reaction is not likely to generate a high enough concentration of radicals to initiate a chain reaction.