The answer is contained in the Kakashi: Shadow of the Anbu Black Ops arc in the anime. Given this question was asked in 2013, that probably explains why none of the answers have the correct answer, as it probably wasn't readily available at the time (I didn't actually check the dates when the episodes came out, I'm just guessing).
Towards the end of the Third Shinobi War, the Third Hokage negotiated for a peaceful resolution without reparations, and this was viewed as weakness by many. The Third Hokage was blamed for the losses of the war, and Hiruzen stepped down as Hokage to appease everyone who was annoyed with him and everyone who was angry with him. This is his way of accepting responsibility. Though if you believe Danzo and Orochimaru, Hiruzen Sarutobi intended to use Minato Namikaze to basically continue his reign, so stepping down was somewhat of an empty act to get everyone off his back.
From the Fandom wiki:
In Konoha, although the war had been proceeding favorably for the village, the Third Hokage established a policy of reconciliation to bring an end to the fighting with an unprecedented offer to not seek reparations from Iwa. Advocates for the war opposed Hiruzen's seemingly weak decision, and to keep dissatisfaction in the village in check, he decided to step down as Hokage.