According to the qualification needed for M.Tech a student needs to qualify the GATE exam. A senior of mine was the first one in my knowledge to join M.Tech CS. She ranked within the top 300 in GATE electrical engineering.
Gradually mechanical and other core branches are also allowed to apply for M.Tech in CS. It is also mentioned in the GATE brochure of ISI Kolkata that graduates from other branches can apply to M.Tech in CS
Gradually a lot of my seniors applied for M.Tech in CS after completing their graduation. A survey says that major of the applicants for M.Tech in CS are from other branches like mechanical and core branches also.