I've wondered about that too. One preacher I heard speculated that perhaps He started writing the names of the men there who were going to stone the woman and the names of the women they had committed adultery with and the dates.

Someone else I heard linked Jeremiah 17:13 to this incident, which says:

"Lord, the hope of Israel,

all who abandon you

will be put to shame.

All who turn away from me

will be written in the dirt,

for they have abandoned

the Lord, the fountain of living water."