Explanation:

There were a few major events that changed the currency rates.
1) June 4, 1966. First major devaluation. For the first two decades, India had almost a constant peg against the dollar at Rs.4.75/$. Then things changed in 1966. India had just fought 2 major wars (with China and Pakistan) and have had 3 prime ministers in 3 years (Nehru, Shastri, Indira) after 17 years of one man rule. Then a major drought shook the country.
2) 1980s inflation. From 1966 to 1980, rupee stayed constant. However, the 1979 energy crisis and gold's skyrocketing prices in early 1980s left India with no place to go (oil and gold were historically our primary imports). Indian rupee started to slowly decline.
3) 1991 crisis. In July 1991, we hit another major crisis. Biggest event in modern Indian economic history. What exactly happened to the Indian Economy in 1991 in layman's terms? Overnight rupee was devalued by another 50% from about 17/$ to about 25/$.
4) 1993 liberalization. In 1993, Indian finance minister Manmohan Singh let rupee to float a little freely. Translation: the rupee was allowed to be traded by traders without a forced peg such as the one kept by China.
5) 1997 Asian financial crisis. One of the biggest events in East Asia as economies such as Thailand and Indonesia collapsed.
6) Pokhran-II 1998. Indian Prime Minster announced the nuclear testing. US, Japan and other countries immediately imposed sanctions on India, limiting investments.