- Her own lack of useful knowledge as regards her own body means that frigidity and hostility are the only reason why she doesn't have a litter of irritating and incompetently raised children and she assumes that Mike will be as bad at not oopsing someone as she is.
- She has a lot of stored-up rage from her teenage years directed at prettier girls who dressed and behaved better and wants to punish Martha in the here and now for a contemporary 'stealing' a boy who just wasn't into her.
- Ever since construction workers stopped whistling at her, Elly has been convinced that she and rampaging senility are BFFs. This means that any sign of maturity on her children's part means that they're inscribing her grave marker right the Hell now because she's old before her time.
What Mike did to set Elly off was make a flippant comment about her obvious belief that he was gonna get Martha knocked up and destroy the family because he does the moron and evil thing called "remembering 'Mom is convinced that me and Sistwirp wanna destroy her or somethin' so we'll always do the worst thing we can just to be mean.'" What he fails to remember is that Elly doesn't want to admit that she sees Martha as a floozy or that she's still pissed off that Colin had a girl or that she thinks that a girlfriend in the morning means being set adrift on an ice floe to die by nightfall. She WANTS to believe that she wants to keep him from getting hurt so much that his telling her the truth about herself leads to actual parenting.