Linda knows that Willy is depressed. He is overworked, he still has to travel for his work, he is working for commission. He has had one car accident that seems suspicious, and he is talking to himself. She has found the hose that he wants to use to kill himself with. She also knows that he is not only disappointed in his own lack of success, but he is very disappointed with Biff. Biff is in his thirties and he still hasn't found himself yet. Willy goes from calling Biff a "lazy bum" one minute to a god the next.
She is most angry at her sons because she does not think they pay their father the proper respect. She wants above all else for Willy to be happy, and she does not want Biff (or Happy) to upset him with the truth. She prefers that they cater to Willy's illusions of their success. When they leave him alone in a restaurant, Linda is furious with them. She does not think they treat him as they should. And she refuses to let Biff talk negatively about him.
Linda desperately wants peace in the family, but she probably sacrifices too much for it, and she is ultimately unsuccessful.