The cutoff switches are operated every flight so the muscle memory is there, ready to be triggered at the wrong time.
All we know is that something went wrong in the pilot's head in at least a single moment that caused him to perform a ground action during takeoff.
Depressive murder-suicide is one possible explanation. Altered mental state is another: insomnia, illness, drugs/medications could all explain an extreme brain fart. Perhaps he just had food poisoning? It's India after all.