Part of the reason for her use of the word "lord" is likely linked into the Symbolic and prophetic nature of Abraham and Sara's relationship.
As with later prophets it is inferred that God uses the life of Abraham and his family to prophetically foretell future events. The symboloy of their marriage relationship is very similar to that of the Prophet Hosea and his wife where by the prophet represents "God" and his wife represents the nation of Israel. The marriage represents the "covenant" agreement they enter into together.
In the book of Hosea this is explicitly stated:
Hosea 1:2 When the lord began to speak through Hosea he said "Go and marry a permiscuous woman and have children with her." For like an adulterous wife this land is guilty of unfaithfulness to the Lord.”
Here it is made explicitly clear that the marriage between the prophet and the permiscuous woman represents the broken covenant agreement between God and Israel. The prophet Hosea represents God - the permiscuous woman the unfaithful Israel. The children they have are named to prophetically describe punishments that would be sent on the future generations as a result of the unfaithfullness to God.
In the story of Abraham it is not explicitly stated but is strongly inferred through symbology that continues through the old testament that Sara - the Barren Woman - represents Israel in the same way as the permiscuous woman did in the book of Hosea. There are recurring referrences to the symbology of Israel as the "barren woman" and Gods wife - such as in Isaiah 54 which explicitly uses this symbology describing Israel as a "barren woman" and his wife whom he briefly abandoned before forgiving and inviting back with compassion.
“Sing, barren woman, you who never bore a child;
burst into song, shout for joy, you who were never in labor;
because more are the children of the desolate woman than of her who has a husband,” says the Lord.
For your Maker is your husband — the Lord Almighty is his name—
the Holy One of Israel is your Redeemer; he is called the God of all the earth.
The Lord will call you back as if you were a wife deserted and distressed in spirit— a wife who married young, only to be rejected,” says your God.
7 “For a brief moment I abandoned you, but with deep compassion I will bring you back. In a surge of anger I hid my face from you for a moment,
but with everlasting kindness I will have compassion on you,” says the Lord your Redeemer.
Once we understand this symbology it becomes clear that the prophet Abraham just like Hosea is playing the role of "God" and Sara the role of Israel. And the key events that take place in their marriage are prophetic foretellings of things that will take place between God and Israel and thus her referring to him as "lord" likely is linked to Abraham in the story is essentially "role playing" God.
Abraham handing handing Sara over to the Egyptian Pharoah in Genesis 12:10-20 is a prophetic foretelling of the events that take place several hundred years later in the book of Exodus. Where God hands Israel over into slavery in Egypt then Moses brings them out with great posessions.
Abraham handing Sara over to the King in Phillistine/Babylon in Genesis 20:1-18 is a prophetic foretelling of the Babylonian Exile which happens around 1500 years later which begins in the book of Jeremiah 39.
Then lastly we have the birth of Isaac and his mock sacrifice on the mountain which prophetically foretells when Israel would bring forth a true spiritual son - born of the Isralite blood line and Gods holy spirit. Jesus / Yeshua. The "barren woman" of Israel gives birth. The mock sacrifice of Isaac Abrahams son symbolically prophecising Jesus future atoning death on the cross.
Short summary - Lord is likely used because Abraham is prophetically role playing "God" over the course of his story in Genesis and Sara role playing "Israel".