Why did Sarah demand to expel Hagar?

Question

IIRC, Hagar was [happily] married to Abraham for at least 17 years (one or two for pregnancy, 13 of Yishmael before his circumcision, one for Sarah's pregnancy, and two for Itzhak's Gmilah). If it was for Hagar, Sarah could demand expelling her right after Yishmael's birth.

If for Yishmael's wickedness, why expel Hagar and why the verse puts Hagar first:

"She said to Abraham, “Cast out that slave-woman and her son, for the son of that slave shall not share in the inheritance with my son Isaac.” Genesis.21.10

when it should be reading "Cast out that boy and his mother".

From the fact that Abraham married her back and had more children with her, presumably following God's command to follow Sarah's prophecy I can infer that Hagar wasn't connected to Yishmael's wickedness.

So why would Sarah suddenly demand the expelling of Hagar?

Answer 1

As far the latter part of your question, as to Avraham could remarry her....

It says in Bereishis 25:1

וַיֹּ֧סֶף אַבְרָהָ֛ם וַיִּקַּ֥ח אִשָּׁ֖ה וּשְׁמָ֥הּ קְטוּרָֽה׃

Avraham took another wife, and her name was Keturah.

The Zohar 1:133b writes that Keturah was Hagar and that Avraham divorced her because of Yishmael, at which time she worshipped the idols of her father house. But later she fully repented and now that her deeds had changed, Avraham sent for her and had her name changed from Hagar to Keturah.

So at least as a starting point to your question, Avraham deemed that she had fixed her deeds enough to warrant remarriage and a change in name.