The Duchess of York is Richard's mom. She doesn't exactly have a great relationship with her son. She even curses Richard to "die by God's just ordinance"(4.4.17). But really, who can blame her? The guy's responsible for the deaths of two of her children and has made his mother's life a living hell since the moment of his (premature) birth. Seriously. Check out this passage where the Duchess lays into Richard:
Thou camest on earth to make the earth my hell.
A grievous burthen was thy birth to me;
Tetchy and wayward was thy infancy;
Thy school-days frightful, desperate, wild, and furious,
Thy prime of manhood daring, bold, and venturous,
Thy age confirm'd, proud, subtle, sly, and bloody.
More mild, but yet more harmful, kind in hatred:
What comfortable hour canst thou name,
That ever graced me in thy company? (4.4.15)
Wow – that's pretty harsh. Not even Hamlet's mom told her son he was sent to earth to make her life a living hell, or complained that his birth was a grievous burden. We're not exactly shocked to hear that the duchess hates her son; we're just wondering why it took her so long to publicly turn against him.