The translation says that the concubine was unfaithful to him.
There is some discussion on what the word really means. It is a fairly broad term (זָנָה) that covers sexual immorality as well as a more general abandonment. So was she playing the harlot (see NASB, NKJV, NIV)? Or was she angry with him (NRSV, NET, NLT)? This is how some ancient versions have it, such as the Septuagint and the Targum. In this case the Hebrew text’s זָנָה is either a scribal error for זנח, to reject, detest,
or it is the original word, which may be derived from a root meaning to be angry, quarrel.
1 This latter reading seems the more likely, not least because it fits the Levite’s behaviour as a calloused and abusive man. Something happened to cause estrangement in this marriage, where she walks out on him in anger. That was then her unfaithfulness, since Israelite law did not allow for a woman to divorce a man. Nevertheless, she left him and returned to her father’s home. The text does not explicitly blame either party for the separation, but in light of the Levite’s later conduct, it seems more likely that he is the one who provoked the separation.2
2 And his concubine was unfaithful to him, and she went away from him to her father’s house at Bethlehem in Judah, and was there some four months.