Rashi citing Sanhedrin 21a: She tied a hair [over her opening at the time of Bi'ah], and it castrated him.

Radak: This was from Shamayim, to increase her shame, and cause Avshalom to hate him to the point of wanting to kill him. Just like David did an act of Zenus that led to [Uriyah'] death by the sword, so he was punished here, and also Avshalom did Zenus and fell to the sword.

Ralbag: She tried to stop him as much as she could. Perhaps she pained him during the struggle, or verbally disgraced him. She already said "you will be like one of the vulgar in Yisrael." Perhaps she later said worse than this!

Malbim: The love was not of her herself; it was animalistic lust. Once the lust extinguished, he realized the abomination that came from it, and hated the entire matter that led to this. "Gedolah ha'Sin'ah