There were many disputes concerning the Palestinian PLO, not only with Christian Maronites but with pretty much all other groups in Lebanon.
Back then it was still the cold war and groups the Socialist Progressive party and the Communist party and the early Syrian national party which was a pan-Arab party calling for union with Syria, We even had Baathists for some while. All those were supported by eastern powers mainly the soviet union and post 79 Iran joining in with the support. Basically all the leftist groups were supported.
On the other hand you had Maronites-(And note here that it wasn't all Christians but literally just Maronites as other Christians were mostly Left wing)- whom were holding the government as the law makes it that the presidency is always a Maronites (Prime minister- Sunni, Speaker of the Parliament- Shia, Head of Judicial authority-Druze. Etc), and the Maronites were pro-western capitalists , which meant that there were heavy political tensions between Left wing and right wing groups in the country for the decades following the 1958 crisis.
Then comes black September, in which many palestinians along with the PLO leaves Jordan and moves to Lebanon. This led to a major imbalance between the left and right wings, and so, just like in 1958, this imbalance started reflecting on the political stage heavily and tensions began rising between the various groups and in a couple of years (early 70s, pre-civil war) a series of Assassinations, small armed engagements and conflicts, etc all started to happen across the country. And the Palestinians, naturally, joined in the fighting.
In 1975, an agreement was made to make sure all hostilities were stopped between all parties, this agreement reformed the authorities of each position of the government and made it much more interconnected instead of each sect controlling a section of the government with no regard to the rest, it decentralized some of the executive powers of the government, infact even the semi-autonomy of Baalbak is a result of it since on its basis Baalbak will latter on become a semi-autonomous city after a deal with Emile Lahoud. Ofcourse the most major and important outcome of this agreement was that the PLO, under international sponsorship ( the UN was also involved) would get authority over the Palestinian refugee camps.
This as you might imagen really pissed off the nationalists whom saw this as assault on Lebanon's sovereignty (and it was rightfully so), and in mid-year, a shooting happened near a Church in Ain Al-remmaneh (the gunmen, to this very day, were not identified and none of the parties involved in the conflict took responsibility) killing several people including some members of Al-kataeb, i.e members of the Lebanese Phalanges, which ofcourse the Kataeb accused the Palestinians, whom they were already angry at after the agreement, and retaliated with the buss massacre only hours latter.
This would evolve into a small conflict called the War of the hotels which would be represent the beginning of the civil war as the Palestinians at that point became fully under Yaser Arafat's control and thought instead of living side by side with the rest, they want full control, and basically war.
The war of the hotels would last well into 1976 and include both the Karantina and Damour massacres to be considered part of it.
Basically, in summary, the Palestinian PLO though started the civil war, was more of an outside movement that tripped into an already missed up political climate; rather than, as some believes, creating it. (like how the US tripped into the Iraqi sectarian battlefield
And ofcourse the main affect the Palestinians played in the war was giving rise to the ultra-nationalist right wing parties like Al-kataeb (Lebanese Phalanges) , and the Morabitoun (no idea what they call them in English
), and the militarized militias of Al-Quwat (Lebanese forces)
With their attempt to take over the country, they revived literally all the tensions and barried disputed since all the way back to the 1800s. I mean seriously, you had leaders going back to mentioning the 1860 conflict between Maronites and Druze and using it as justification, thats how far the PLO have revived of Lebanon's old disputes.
And then you had the third stage of the war the Israeli and Syrian invasions and occupations which further exacerbated the divide as multiple parties as you know started taking the side of one or some times both of the foreign armies in order to defeat an internal enemy being another groups or political party.
And this only ended in 2007 after a new agreement was made between all sides in regards of how the country should be run. Not in 1991, or 1989, nor in 2000 with the Israeli withdrawal or in the 2005 Syrian withdrawal.