Bam Margera's wife says a specific incident triggered her to file for separation ... and we're told it was Bam spending time with their son while, she believes, he was under the influence.
Nikki Margera's attorney David Glass tells TMZ ... her recent filing came after the 'Jackass' star visited their 5-year-old, Phoenix, while Nikki was present.
We're told she thinks Bam wasn't sober -- despite him claiming he was -- and she says he was acting inappropriately.