SEOUL, South Korea — Did a fight over a puppy lead to the vote to impeach South Korea’s leader? A whistle-blower whose account has riveted South Koreans says it did.

The corruption scandal that engulfed the president, Park Geun-hye, was rooted in her friendship with Choi Soon-sil, her secretive adviser and friend for 40 years. Ms. Choi has been indicted on charges of extorting tens of millions of dollars from big businesses, and prosecutors have identified Ms. Park as a criminal accomplice, a first for a South Korean president.

Ko Young-tae, the whistle-blower, said at a parliamentary hearing this week that he got into an argument with Ms. Choi over walking her daughter’s puppy one day in 2014. He said Ms. Choi became upset when she found the dog alone at his home.

“I didn’t get her call because I was out golfing,” he said. “She was angry that I left the dog alone, and we had a fight.”