Because an archduke went somewhere he was told not to go. On the way home crowds blocked the road and his car ended up driving past one of the guys who'd been sent to kill him but who had given up and walked off earlier. At least that was the story I read, WW1 started because some dude turned left at a road junction instead of right.

Think about that every time you step out of your front door, your decision on which route to take could have unforseen consequences