Why was Yemen split into 2?

In 2015, Yemen had a problem of some people supporting the current president and some supporting the former president. Iran and Saudi Arabia have also gotten involved.

http://www.bbc.com/news/world-middle-east-35160532
The Yemeni government was fighting with the Saudi Arabians to declare their independence they merged in the 1980's they are thinking of re-splitting due to a national war between the two nations.