Yahoo announced hack day in Hyderabad due to a tremendous response from the developer community in India. This year Yahoo seems to have planned to conduct events in different cities starting with the Hack Day in Hyderabad followed by a Hack Day (Yahoo Open Hack 6) in Bangalore.
Yahoo Developer Blog said
Yahoo! Hack Day in India has been a cherished tradition for Yahoo! Developer Network since 2007. We have hosted some of our largest hackathons in India and have been humbled by the overwhelming demand (last year, more than 4,000 applied) from top developers to participate year after year. During our past hackathons, we have heard stories of developers traveling from several different states around the country to hack with us. We know that India has a number of fast growing tech hubs with brilliant developers in addition to Bangalore. So this year, we are thrilled to expand our hackathons to a new location with the first ever Yahoo! Hack India in Hyderabad on July 13-14! Later this year, Yahoo! Hack India will return to Bangalore to reunite with our old hacker friends, and we will continue to take our hackathons to other Indian tech hubs.