MTV Star Johnny Bananas with John Elway, Agent During Golf Cart Accident

MTV reality star Johnny Bananas was riding with John Elway and Jeff Sperbeck during the fatal golf cart accident, with sources telling TMZ Sports ... "The Challenge" competitor tried to keep the agent alive after he fell off the vehicle.

We spoke to someone familiar with the incident ... who tells us, Bananas -- real name Johnny Devenanzio -- and Sperbeck were standing on the back of the cart as Elway drove and his wife sat in the front seat.

We're told Sperbeck lost his balance and fell backward off the cart -- hitting his head on the asphalt around 7 PM.

The source describes it as nothing more than a "freak accident," and is adamant Elway was not speeding or driving erratically on the pavement prior to Sperbeck's fall.

As for Elway's condition while behind the wheel, the source does not believe alcohol played a factor.

We're told Bananas rushed to tend to Sperbeck as the group waited for paramedics to arrive ... and tried to do whatever he could to help before professionals took over.

The Sun first reported Bananas' involvement.

Sperbeck was rushed to a nearby hospital, and passed away from his injuries on Wednesday at 1:10 AM.

As TMZ Sports previously reported, the group was in town for the Stagecoach music festival ... and Elway attended a swanky party at one point in the day before the accident.

Bananas is close with Elway's son, Jack ... and the 2 even posed for pics together throughout the event.