Linda tells her sons to leave because she wants to defend her husband from the emotional harm she sees looming in a another confrontation with Biff. She wants to save Willy from self-destruction and to keep her sons from leading Willy in that direction.
Willy is not in a good state of mind. He is in a poor state of mental health and Linda sees this clearly. She has found the rubber hose in the basement and she knows how fragile Willy is, mentally and emotionally.
She feels that Biff and Happy are not helping Willy to recover. They are instead aggravating his condition, making him more nervous, more violent in his temper. They are moving him toward the edge.
Biff and Hap’s insensitive behavior, she fears, will push Willy closer to despair and suicide.
While she worries that Willy might be pushed to take his own life, Biff and Happy do not know how to treat their father. Their behavior strikes Linda as completely uncaring. Her concern is spiked when they return from the restaurant where they were to meet Willy to celebrate the loan that Biff did not receive.
...Biff and Hap enter late at night, finding Linda outraged at them for abandoning Willy at the restaurant.
At the heart of Willy's weakness is a profound self-doubt. Willy worries that he is a failure. Linda recognizes this. She sees also that Biff seems to want to tear down his father.
Linda violently asks Biff whether he cares if Willy lives or dies.
Willy needs help, not an emotional challenge from his favorite son. Happy does not help either. He matches Willy's weakness with his own need for attention, further diminishing Linda's energies, which are required to make a last effort to save her husband.