As if to signify that such persons were to be written in earth, not in heaven, where He told His disciples they should rejoice they were written. Or His bowing His head (to write on the ground), is an expression of humility; the writing on the ground signifying that His law was written on the earth which bore fruit, not on the barren stone, as before.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Con. Evang. lib. ii. c. 10