USADA says they investigated him because -- get this -- he posted an image of himself receiving an infusion on social media (appears to be Instagram). Now he's banned for 14 months.
Here's what USADA said in its release (the actual link on the USADA site isn't working right now):
"USADA announced today that Ryan Lochte, of Gainesville, Fla., an athlete in the sport of swimming, has accepted a 14-month sanction for his use of a prohibited method.
"On May 24, 2018, Lochte, 33, posted an image on social media depicting himself receiving an intravenous infusion. A subsequent investigation by USADA, with which Lochte fully cooperated, revealed that Lochte received an intravenous infusion of permitted substances at an infusion clinic in a volume greater than 100 mL in a 12-hour period without a Therapeutic Use Exemption (TUE).
"Intravenous infusions or injections in a volume greater than 100 mL within a 12-hour period are prohibited at all times – except for those legitimately received in the course of hospital treatment, surgical procedures, or clinical diagnostic investigations under the USADA Protocol for Olympic and Paralympic Movement Testing, the United States Olympic Committee National Anti-Doping Policies, and the Fédération Internationale de Natation Anti-Doping Rules, all of which have adopted the World Anti-Doping Code and the World Anti-Doping Agency Prohibited List."