The Secret Service had to 'call an audible' during Hillary Clinton's exit from a 9/11 memorial

Members of the US Secret Service followed "atypical protocol" during Hillary Clinton's abrupt departure from a September 11 memorial on Sunday, according to The Washington Post.

Video captured at the New York City event showed the Democratic presidential candidate struggling to keep her balance as she is helped into a black van. Aides said Clinton felt "overheated," and her personal doctor later revealed Clinton had been diagnosed with 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 is likely due to 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. However, an agency spokesman said in a statement that "at no time did any Secret Service personnel violate security protocols."