This is because when Elly reacts to a non-event as if it were a catastrophe, she's saying "No" to interpreting things as being harmless. She has to see everything that alarms, inconveniences or confuses her as being part of a plot to destroy her not only because of an innate belief that everything will always work out for the worst but because she's a creepy narcissist with a persecution complex.
This is why she's also saying "No" to things not being a big deal. After all, if every little thing didn't mean that the world was actually coming to an end, she'd have to say 'Yes' to being a melodramatic idiot with a need to monopolize the conversation at everyone else's expense and she just can't do that.
What she's also doing is saying "NO!!!!!" to understanding how people really feel and how they see her. In the example I linked to, she goes on her merry way not realizing how horrible Lizzie felt about the whole thing because understanding that would mean saying "Yes" to the idea that her children fear her incessant and baffling rages.
Given that this happens time and again, she's saying "No" to remembering the past. You see, remembering the past might tell her that the world hasn't come to an end yet despite what the children might do. It would also tell her that despite her current belief that the children who she always got along with have suddenly become a problem have always been a problem. Since that would mean that she might be the problem, she wants to live in an eternal present.
She's saying "No" to a realistic understanding of the consequences of her children's actions. After all, if Mike doesn't take his trumpet lessons seriously, he isn't really going to die in a gutter and bring shame on her or any of the horrible things she thinks are certain to happen if he just wants to blow off steam, race around like a goofy kid or try to have a little fun with his trumpet. If the world isn't really going to come to an end, she's a chump who thinks the world has to come to an end because she doesn't have it in her to be happy.
She's even saying "No" to the realization that her children can only make her upset with her own express permission. Time and again, she forgets that the only reason her kids like to push her buttons is that she witlessly gives them what they need: a mother only too willing to destroy her credibility by throwing a tantrum in the name of preserving said credibility.
Since it's next-to-impossible for her to admit that she's a stupid fool who confuses wallowing in dimwit cynicism with being wise to the ways of the world, she's especially saying "NO!!!!!" to any idea that people aren't spending their days trying to hurt or disappoint her because they hate her. They'd have to respect her first and since she says "No" to making herself a force to be reckoned with, all she gets is pity and contempt.