Answer by taking a quote from the following article:

Jean-Marie Le Pen (French pronunciation:  [Za ma.Ri l@.pen]; born 20 June 1928) is a French politician who has served as Honorary President of the National Front since January 2011 and a Member of the European Parliament (MEP) from France since 2004, previously between 1984 to 2003. He previously served as President of the National Front from 1972 to 2011.

Prosecution for assault : In April 2000, Le Pen was suspended from the European Parliament following prosecution for the physical assault of Socialist candidate Annette Peulvast-Bergeal during the 1997 general election. This ultimately led to him losing his seat in the European parliament in 2003. The Versailles appeals court banned him from seeking office for one year.  Statements about Muslims in France : In 2005 and 2008, Le Pen was fined, in both case 10,000 euros for "incitement to discrimination, hatred and violence towards a group of people", on account of statements made about Muslims in France. In 2010. the European Court of Human Rights declared Le Pen's application inadmissible.  Allegations of war crimes in Algeria : Le Pen allegedly practiced torture during the Algerian War (1954-1962), when he was a lieutenant in the French Army. Although he denied it, he lost a trial when he attacked Le Monde newspaper on charges of defamation, following accusations by the newspaper that he had used torture. Le Monde has produced in May 2003 the dagger he allegedly used to commit war crimes as court evidence.  Although war crimes committed during the Algerian War are amnestied in France, this was publicised by the newspapers Le Canard Enchaine, Liberation, and Le Monde, and by Michel Rocard (ex-Prime Minister) on TV (TF1 1993). Le Pen sued the papers and Michel Rocard. This affair ended in 2000 when the Cour de cassation (French supreme jurisdiction) concluded that it was legitimate to publish these assertions. However, because of the amnesty and the statute of limitations, there can be no criminal proceedings against Le Pen for the crimes he is alleged to have committed in Algeria. In 1995, Le Pen unsuccessfully sued Jean Dufour, regional counselor of the Provence-Alpes-Cote d'Azur (French Communist Party) for the same reason.

What was the reason for the physical assault of Annette Peulvast-Gergeal?
the physical assault of Socialist candidate Annette Peulvast-Bergeal during the 1997 general election. This ultimately led to him losing his seat in the European parliament