By some distance the most popular mode of death for Who companions. This is probably because, as well as delivering the emotional gut-punch of seeing a beloved character die, it holds your hand afterwards, pats your head and says, Shhh - at least they died for a reason. River Song did it (electrocuted while saving everyone), Astrid Peth did it (jumped into a nuclear-powered spaceships engines while saving everyone), K9 Mark III did it (exploded some nasty child-exploiting bat-things led by Giles from Buffy the Vampire Slayer), Lucie Miller did it (flew a Dalek ship into the Earth's core), C'Rizz did it (melted his own insides to bits while hosting the last vestiges of a dying species in his body), even the Doctor himself did it (took a highly radioactive shower while releasing poor old Wilf Mott from a plexiglass prison of his own making): the list goes on and on. It gives the dying companion a final, noble scene to end on, and the actor playing the companion gets a chance to crack out their best stoic face. Actors absolutely love doing their best stoic face. They think it gives them a chance to let casting directors see what it'd be like if they were acting in something proper, like Shakespeare or a period drama with Jeremy Irons in it. In fact, Clara herself has already sort of done this too, swan-diving into the Doctor's timeline to save him in various times and places throughout his life without his knowledge. She's certainly got the experience and lack of regard for her own wellbeing for this to be a distinct possibility.
