(ことになった) When you did not expect it to be happened.(got the news recently) Quando você não espera que ele seja aconteceu.(recebido a notícia recentemente)

海外に行く=I'm going to abroad. (Basically you decided to go) 海外に行く事になった=sounds like "you need to go or you have to go. (Someone send you abroad. It occurs especially as a business trip)

I hope someone answers this so I can also know ^^