The recurrence of this sin is noted by the twice daily offering of Ketores . On Yom Kippur , something more than the average Ketores was needed. It affects the majority of people. Thus, Klal Yisrael must atone for it on the day when everyone stands in prayer begging for forgiveness. Avak lashon hora is the source of most sin.
Many sins begin with a simple derogatory remark about someone which becomes magnified over time.
Horav Simcha Zissel Broide, Shlita, notes the compelling nature of this statement. We are taught that Rav Chanina ben Dosa was once immersed in prayer. So profound was his devotion during tefillah that when a snake bit him, he remained unaware of it. Ostensibly, Aharon Ha'kohen manifested a similar level of devotion during his encounter with the Al-mighty, especially on Yom Kippur in the Kodoshei Ha'kodoshim . It is easier to ignore the physical pain of a snake bite than to disregard the demands of one's middos , character traits.