This was needed to pass a new Abortion law which the cabinet and parliament had passed. The Catholic King refused to sign the law due to religious reasons. According to the constitution, laws need to be signed by "the head of state" which is technically the King. The cabinet proceeded with "treating" the king as "unfit to govern", making the MP the head of state and the entire cabinet signed the law into effect. After 36 hours everything was back to normal and the Belgians stopped "treating" the King.
In 1990, when Baudouin refused to sign into law a bill permitting abortion, the cabinet assumed the power to promulgate the law while he was treated as "unable to govern" for twenty-four hours.
https://en.wikipedia.org/wiki/Baudouin_of_Belgium#Notable_events
The civilian Government temporarily suspended King Baudouin I from power today after he declared that he could not, in good conscience as a Roman Catholic, sign a new law permitting abortion.
After declaring the King ''unable to govern,'' the Cabinet assumed the King's powers and promulgated the abortion law, which was published in the official gazette. And it called the Chamber of Deputies and the Senate back from Easter vacation for a special session on Thursday.
http://www.nytimes.com/1990/04/05/world/belgian-king-unable-to-sign-abortion-law-takes-day-off.html
Dutch language sources:
https://nl.wikipedia.org/wiki/Abortus#Wetgeving_in_Belgi.C3.AB
http://deredactie.be/cm/vrtnieuws/binnenland/1.749701
http://nieuws.vtm.be/binnenland/135916-25-jaar-abortuswet-boudewijn-onder-druk
https://www.nrc.nl/nieuws/1990/04/04/boudewijn-treedt-tijdelijk-terug-6927041-a47243
@jidlaph we where having a discussion about royal powers in Europe