India dropped in the ladder largely because the record of 12 Tests victories and just one Test defeat in 2016-17 was removed in the latest chart, the ICC said in a statement. Virat Kohli's men had won all five series during that period including against Australia and England.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/news/india-lose-top-spot-in-icc-test-rankings-to-australia/articleshow/75486448.cms