Original: Under UCI rules, the determination of whether or not a cyclist violated any rules must be made by the cyclist's national federation, in this case USA Cycling, which transferred the case to the United States Anti-Doping Agency (USADA).
Simplified: Under UCI rules, the cyclist's national federation, in this case USA Cycling decides if the rules have been broken, but because of the science involved they asked the United States Anti-Doping Agency (USADA) to help.