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
Why did Sarah torment Hagar (by increasing her workload)? Was it an act of revenge?
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 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.
רש"י: היתה משעבדת בה בקושי: ממה בא רש"י להוציא?
גור אריה: אין לפרש שייסרה אותה כי לא כתוב 'ותייסר אותה' אלא הייתה משעבדת אותה בקושי.