The Lebanese president invited them. In 1976, a little more than a year after the start of Lebanon's bloody civil war, the Maronite Christian president, Ilyas Sarkis, requested military aid from Syria. The war was a struggle among religious and ethnic groups in Lebanon—the ruling Christians wanted to maintain power over the Sunnis, Shiites, and Druzes, and to evict the large population of Palestinians that had just been expelled from Jordan. Syrian troops entered the country on June 1, 1976, and beat back both Palestinian forces and Soviet-backed Muslim militias. A few months later, the Arab League attempted to enforce a cease-fire by creating the Arab Deterrent Force, a Syrian-dominated military presence in Lebanon.
That article indicates that the Christians and Syrians had a split in 1978. Is there anything to indicate recent support for Syria by the Lebanese Christians?
I doubt that the fate of the South Lebanese Army when Israel withdrew and the Syrians-Hezbollah took over in the south endeared the Syrians to Maronites very much.