What set Mel off...

Hours before Mel lost his cool on the phone on February 18, he and Oksana were in the backyard of his Malibu mansion ... watching as a gardener planted a Santa Lucia fir tree in honor of their daughter Lucia.

But that's not all that went in the ground that day -- sources close to Oksana tell us the couple also buried the placenta from the birth of their then 3-month-old daughter Lucia ... a tradition in some parts of Australia ... where Mel grew up.

Mel also did this placenta ceremony for all of his children.

The sources also says during the ceremony the gardener made an innocent comment and Oksana smiled at him. We're told that's what set Mel off.

As we first reported, Mel then accused Oksana of having an affair with the gardener ... and that's when all hell broke loose.