What would have to happen for Earth's oxygen to decrease at an alarming rate and furthermore be dangerously low or completely gone in a year?
Stop producing oxigen.
Oxigen is so reactive, you can't have a supply without something that releases it constantly. Hind plants and plankton hability to release oxigen and the numbers will fall down fairly quick. UV blocking material clouds, a virus that breaks the clorofila reaction... you name it. Stop the engine, and the factory falls silent.
Scorched Earth - literally. I'm not quite able to do all the math, but I'm pretty sure that if all the biomass on our planet were consumed in a huge fire, most if not all the oxygen in our atmosphere would be converted to carbon dioxide. I also think this is likely to be one of the fastest ways this could happen.
A massive release of Iron in oceans, via e.g. tectonic processes.
Iron binds oxygen, forming iron oxides (that's how we get iron ores). This consumes a lot of oxygen.
Augment it with carbonate-bicarbonate process that flushes carbon monoxide out of atmoshpere and into the ocean, and plants don't have enough CO2 to replenish oxygen with.
Without going the evil aliens route a flashburn of the planet would be the "easy way" to do it. Otherwise the only thing that could crash O2 would be a meteorite made of something really reactive grazing the atmosphere like Ruthenium+8 that sucks up oxygen at a rate of 4-to-1 but it would have to ablate megatonnes of material into the atmosphere to make an appreciable difference.