Farrah and Amber will clash on the upcoming Teen Mom OG reunion special -- but what was the boiling point that sparked the all-out feud between the young women? It's the calm before the storm...

In a sneak peek from the dramatic installment, below, host Dr. Drew asks Farrah's on-again off-again boyfriend Simon about some recent Snapchats which featured the SoCal-based entrepreneur speaking negatively about his fellow MTV cast members (the addiction specialist asked Amber about the same social media updates during Part 1).

"These people make themselves vulnerable -- they give you ammo to just talk about them," Simon explains, while adding that he means "no disrespect" and that it "was all in fun."

Meanwhile, the 25-year-old defends her friend's actions -- but doesn't mince words when comparing him to her longtime cohorts.

"He's not being an Amber or being a Maci or being a Catelynn and saying you're going to beat somebody's ass, you're going to create fights," Farrah explains. "Simon's not even doing that."

But the ill feelings only intensify when Dr. Drew asks Simon about calling Amber's fiancé Matt a "pedophile." Hear how the former couple responds to the query in the video below, and don't miss the entire reunion -- unprecedented fight included -- on Monday at 9/8c.