It was 1200 years since the creation and launch of CIN (Collective Information Network) by Google Inc. Having run out of things to search, Google had designed a human-integrating chip, which when installed on a a persons cerebral cortex, allowed the user to both search the internet as well as indexing the persons memories and data banks for fast retrieval on the network. Billed to the impovished as a free alternative to college, it was a huge hit. But after the eventual takeover of Google by Steve Jobs in 1021, And with it full integration with the iPhone6, it became a sensation, even amongst the rich and famous. 500 years later it would become global mandate for these to be installed in all children at time of birth (as it is claimed to be safer to install on a child). The human android race was born, and CIN became the global knowledge bank for a new computerized government.

This of course angered those of the federation, who no longer could view the human species as something to be protected. Linked consiousnesses had been banned for as long as they could remember, and its drawbacks well documented. Since the invention, science was evolving at an unheard of rate, and it wouldent be long before the earth technology could rival that of the federation. They had to be stopped now.

Unsure of how to approach the situation, they disguised several interstellar warships as giant iphone's and approched earths orbit. It is to this day still unknown who's bright idea it was to attempt to download Moris ALbert's "Feelings" onto said warship/iphone, but the devistation that insued is forever ingrained into the minds, iphones, and memory chips of what is left of earths now completely android population.

Asimov's method: The Earth steadily increased in radiation over 100 years, due to a device set off by a radical colonist. Earth became progressively more unhabitable, but everyone didn't instantly die, so it forced them off the planet, which gave the colonies a huge boost in population (the colonies were geo-formed planets in this case, not space stations... it'd be hard to get the 20 billion of Asimov's earth into space stations).

Alternatively, you could have the Tetonic plates of the planet all shift at nearly one time, having built up power over centuries, causing catastrophic earthquakes (Megathrust earthquakes) and tidal waves, volcanic eruptions, and wildfires, which would cause massive famines, wars, and plagues like you couldn't possibly imagine, killing off 80-90% of the Earth's population over a peroid of 7 or 8 years...

If you have a space faring civilization that regularly colonises other worlds and builds many space stations, there is no need to destroy earth, as many people will leave seeking employment and excitment elsewhere in space. Earth might also be a hideously expensive place to live now too.

I would go with the real reason the Earth will no longer be around... in 2012, the magnetic poles will reverse, completely flipping, which will cause the entire planet to flip back to its comfort zone. It will do so, so violently, that anyone not nailed down will be flung into space. Anyone left will be surely crushed by the force of their own inertia when the Earth suddenly stops after its half rotation.