Democratic presidential nominee Hillary Clinton leaving the home of her daughter, Chelsea Clinton, on Sunday in New York City. Getty Images/Justin Sullivan Members of the US Secret Service followed "atypical protocol" during Hillary Clinton's abrupt departure from a Sunday memorial service for the September 11, 2001, terrorist attacks, according to The Washington Post.

Video captured at the New York City event showed the Democratic presidential nominee struggling to keep her balance as she was helped into a black van. Aides said Clinton felt "overheated," and her personal doctor later revealed Clinton had been found to have pneumonia on Friday.

According to The Post, it is unusual for Secret Service agents to allow a subject to wait for a car to arrive, as Clinton did while standing on a curb on Sunday.

It's also unusual for the leader of the detail to leave the subject's side. In the video, detail leader Todd Madison was seen leaving Clinton's side to open the doors of the van.

The atypical behavior was most likely a result of the unexpectedly rushed exit, a former agent told The Post.

"Every once in a while, you have to call an audible," he said. "We follow a methodology, not a script."

The Post originally reported that the agents may have broken Secret Service protocol. An agency spokesman, however, said in a statement that "at no time did any Secret Service personnel violate security protocols."

—U.S. Secret Service (@SecretService) September 11, 2016

—U.S. Secret Service (@SecretService) September 11, 2016