Brandon's wife, Noonie, tells us his heart stopped while he was sleeping in their Northern California home. We're told his death was completely unexpected -- he didn't have any previously known health conditions.

Our law enforcement sources say an autopsy was performed and there are no signs of foul play ... or any obvious cause of death. Brandon's family and authorities are now waiting on a toxicology report.

The internet comedian, aka Young Busco, rose to fame in 2015 when he mocked a police officer's shoes as a joke. While his friend was being arrested, Brandon mocked the cop's clunky boots, saying ... "What are those?"

Brandon uploaded the clip and it quickly went viral and became a popular internet meme. The catchphrase appeared in "Black Panther" and was jokingly asked to Michael Jordan during a Q and A session at a basketball camp.

Brandon, a father of 9, was just 31.