The Indian Government has issued new rules about when a tourist can re-enter India on a multi-entry tourist visa.
As of 2010 anyone using a tourist visa must be out of India for at least two months before being allowed re-entry. This means that a person visiting south Asia on a multi-country itinerary cannot simply hop in and out of India regularly. They must leave India and remain out of India for at least two (2) months before they can legally re-enter the country.
The reason for what seems like a rather draconian policy change is in fact the November 2008 Mumbai attacks. These were apparently facilitated by two US citizens who used Indian multi-entry visas to come and go repeatedly while planning the attacks and providing logistics for the attackers.
Acording to Wikipedia "In October 2009, two Chicago men were arrested and charged by the FBI for involvement in terrorism abroad, David Coleman Headley (real name: Daood Sayed Gilani) and Tahawwur Hussain Rana. Headley, a Pakistani-American, was charged in November 2009 with scouting locations for the 2008 Mumbai attacks. Headley is reported to have posed as an American Jew and is believed to have links with terrorist outfits based in Bangladesh. In December 2009, the FBI charged Abdur Rehman Hashim Syed, a retired major in the Pakistani army, for planning the terror attacks in association with Headley." "Indian investigators were surprised at how easily Headley had obtained a visa to enter India, a process that is extremely difficult for Pakistani nationals. Headley's U.S. passport, his new Christian sounding name, and the fact that the passport and his visa application made no mention of his prior name or nationality, made it easy for him to obtain an Indian visa from the Indian consulate in Chicago. He also falsely stated on his visa application that his father's name was William Headley and that his own name at birth was Headley, a claim that was difficult to refute since the U.S. passport, unlike the Indian one, does not provide the father's name, and does not require endorsements on name changes by the passport holder."
So the actions of some US citizens who probably helped the attackers while using tourist visas and repeatedly and quickly entering and re-entering India have led to the ending of quick re-entry for all tourists. This also explains why the Indian visa application form asks for details about parents and their nationalities at birth.
Lesson: when planning your Indian and South Asian itinerary, remember to have all of India done in one visit, then exit the region through another country.