Safelife,
every reasonable -read: modern- aircraft flies in VNAV these days.
As Suninmyeyes says, be it in VNAV or any other cruise mode, you have to wind down the altitude in the altitudewindow AND select some sort of descent. Meaning, you need not one but TWO actions before the airplane will go down! Unless of course the vnav profile asks for a descent, in which case the airplane will descend to the altitude selected in the window. But they were in the middle of the cruise! Thus, unless Jet Airways flies from Mumbai to Brussels without autopilot, I see no way how this could have happened.