The magnetic poles reversed.
Using Hollywood logic, of course. Common misconception is that this would cause seismic events on Earth, hence the volcanic activity and auroras in Washington, the earthquake in Los Angeles, and the electrical grid failure.- Seems to be what they were going for, at least in part. Though there's a few events that couldn't be explained entirely by that, such as the train crash (or why the military is mobilizing so heavily in the first place for what seems more like a combat encounter than relief work), which may have been thrown in to add ambiguity as to what exactly it was. The establishing shot in space could indicate it was some sort of unknown interstellar phenomenon that hit the planet and caused everything.
The unknown event is the Visitation, from Roadside Picnic
There are strange events going on, weather and magnetic forces go weird, and the army mobilizes? It is most likely the first event of the Visitation.The baby is actually Jeremiah's
Sam was having an affair with him before the movie started. It would explain why Jeremiah wanted to get Will out of the picture.This is the event foretold in Take Shelter
Weird weather? Check. Birds flying in odd patterns? Check.This is the event that set off The Road
Weird stuff, no one knows how it happened.