The 33-year-old former Packers star explained what happened at his Tennessee estate ... where he, his wife Aiyda, and their three kids were at home at the time of the incident.

"First and foremost, we are all safe and healthy," Cobb said in an Instagram post. "We got out of the house and I was able to go back in and get our dog, Louie."

Cobb said the blaze started in his garage, and the situation was so dire, Randall thought his vehicles -- including his Mercedes G Wagon (which RC shows was badly damaged) -- were going to blow up.

Despite how intense the fire was burning, Cobb said the Nashville Fire Department went into "swift action" ... and praised them for their courage.

"I can't get the image of the brave firefighter getting into position out of my head; he didn't even have water to shoot yet," Cobb said. "He is a true hero."

Their family home isn't currently livable, so Cobb said he and his fam are staying at a close friend's house for the time being. RC also thanked the Nashville community for their love and support.

He also used the fire as a reminder that the most valuable thing in the world is family. All the material things can be replaced.

"We don't know how much, if anything, will be salvageable, but this is a reminder that nothing is more important than the health of our family."

Randall's wife, whom he married in 2017, wrote in a now-deleted IG post that she believes the fire started with a Tesla charger ... but Nashville FD tells us they're still investigating.

"There was an electric vehicle (EV) in the garage, along with 1 additional vehicle and Investigators were called to the scene to determine the cause of the fire. There were no injuries reported to civilians or personnel," Nashville FD told us.

"The investigation is ongoing and some details are unavailable, as part of the investigation."