It's not totally clear why the crash is happening. It's not at all obvious why anyone even bothered trying to set an iPhone's clock back 46 years.
But we can make a pretty educated guess.
January 1, 1970 is the earliest date you can set your iPhone to. If you turn your date and time settings to manual (please don't do this), and scroll the calendar back as far as you can go (seriously, don't do this), you can only go as far back as January 1, 1970.
That's because Unix time began at midnight GMT on January 1, 1970. Unix time has been counting every second since then. Many gadgets, including the iPhone, use Unix time as the basis for their clocks. Read more on money.cnn.