What did Avraham mean when he told Sarah that "her maidservant was in her hands?"
Seforno: What he meant was that he had not set her free. 1
See Seforno to Bereishis 30:6.
Why did Sarah afflict Hagar (by increasing her workload)?
Seforno: It was to remind her that she was her slave, and that she should treat her with due respect.
Ramban: She acted incorrectly; and so did Avraham by allowing it. 1
Riva: She had verbally afflicted Sarah. Semag says that the victim of affliction may afflict his oppressor, for the latter is not "Amisecha."
Chidushei Agados of Rav Y.S. Elyashiv: Hagar had stopped acting like a Talmid of Sarah, and considered herself to be a teacher. The affliction was to act like a Talmid, e.g. carry Sarah's clothes to the bathhouse.
QUESTIONS ON RASHI
Rashi writes: "Sarah afflicted her (va'Te'a'neha) - she gave her difficult work." What is Rashi explaining?
Gur Aryeh: We should not think that Sarah inflicted physical torture on her, for if so the verse should have used a different verb, "va'Teyaser."