The German ran wide with nine laps to go, handing to Hamilton the victory he needed to secure a third championship.
"It was a gust of wind - a big one," Rosberg said, adding: "I'm serious!
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"Everybody [experienced it] so you can't use it as an excuse, but I understand my mistake now and that's important, for me to understand it."
Rosberg was furious with himself after the race, throwing his cap for second place back at Hamilton after the Englishman had tossed it to him and refusing to take part in the traditional champagne-spraying ceremony on the podium.
The German added: "It was a very unusual scenario which happened only once in all those laps.
"I understand the mistake and that helps me a lot moving forward. I lost a lot of rear downforce as a result."
Rosberg was also angry after the race in America about the move Hamilton made to take the lead at the first corner, describing it as "extremely aggressive".
He said he did not want to "go into details" about the incident, any discussion he would have with Mercedes about it, or whether he would take a more aggressive approach to Hamilton on the track.
Nico Rosberg has blamed a gust of wind for the mistake that enabled Lewis Hamilton to seal the world title at the United States Grand Prix last weekend.