Bees, of course

It was the bees. It had always had always been the bees, no one had bothered thinking about it before.

An entire species of insects -- are they insects even -- that have been ruling over this planet, nayy, blanketed the planet over millions of years doesn't just disappear over  a matter of decades. No it doesn't work  like that. What was clear was that the bees had put a plan into execution, and the first phase of it involved escaping the Earth in various groups.

The fuel source was of course, obviously, as clearly as a cloudless summer day, the honey. What else could they possibly have gathered them for, for all those years. It wasn't for the flavor, for sure, it was just lucky coincidence for humans, but the more fundamental reason was the energy density. You cannot get that high of energy density naturally from plants -- the surface area of leaves for chlorophyll is not large enough to provide energy to be squeezed into that small of an amount. But the bees synthesized all the energy-dense liquids from the plants, concentrated them, and turned bomb. Synthetic slow-burning liquid fuel that would take them to the stars. Well...maybe not the stars but as far away from earth as possible when the attack happened.

The bees had the magneto-detectors on their heads for bearings, but they could also detect minor fluctuations in the field caused by things such as say, a very powerful magnetic drive approaching the earth at very large velocities, causing a flux that could pass as background noise for human devices but was a very obvious natural event for the bees. Anything that was coming in such a hurry couldn't be good for anyone, so the bees decided to take some time off the planet and left a small farewell party that would keep humans in the dark. The farewell party was not a suicide squad, no, they did plan on leaving at the very last moment, but they were among the riskier of the beekind.

When the humans discovered that all the bees were mostly gone, they were sad and confused and blamed only themselves for the catastrophe. Not for a moment did they consider the possibility that it was a signal, indirect as it may have been, for them to consider making backup plans. Meanwhile, the bees chilled in their ultramodern spaceships in low earth orbit, ready to accelerate to a safer orbit in the smallest of alerts. Their predictions had the invaders on a colliding course with the Earth after three months, but if there's one thing you can't predict about interstellar travelers, it's the speed of their ships. Perhaps they had some magic thingamagic thing that would suddenly shave two weeks of their schedule? Or they were saving up all their acceleration fuel for the last end of their journey, and were on a course to continuous acceleration? You really couldn't say. To account for such possibilities, they had chosen to rather potentially blow their cover than be sitting ducks inside the planet. It was just good survival sense.

The humans didn't seem to have a clue. They went about their days as usual.

Ahh the humility and the ignorance, what he wouldn't give to have that carefree monkey brain agan! What he wouldn't give to relieve that life!