The RAT deploys automatically when the system senses an electrical or hydraulic failure. This is pure speculation based on thousands of hours experience in Boeing aircraft but none in the 787:
Was there a large demand on the hydraulic system, such as landing gear deployment or flap extention? Low hydraulic level could cause a momentary loss of pressure.
Was this a fully coupled, fail active, autopilot ILS approach? At 1500 feet the electrical system splits to ensure power to each of the three autopilots in case of a single electrical failure. Did the system detect a momentary system failure?
Both of these could be traced to lackluster maintenance procedures. Design failure??? Nah. This system configuration has been used since the 1980s and the 757, 767, and 777. I’ve never heard of it happening before.
EC
AIR INDIA.............
I had no idea until now that such a system existed and I think it’s reassuring that they have a backup power generation method.