The MW2 nuke alert comes from Call of Duty: Modern Warfare 2, the 2009 game where a tactical nuke can end the match after a high killstreak. The warning alarm became memorable because it signals that the round is essentially over unless time runs out. Over time, players clipped the sound and reused it in montage edits, meme videos and reaction posts as shorthand for disaster, domination or a point of no return. It still works because the audio cue is immediate and easy to recognize.