Adam Clarke on Psalms 78:9

The children of Ephraim - turned back - This refers to some defeat of the Ephraimites; and some think to that by the men of Gath, mentioned 1Chr 7:21. R. D. Kimchi says this defeat of the Ephraimites was in the desert; and although the story be not mentioned in the law, yet it is written in the Books of the Chronicles, where we read, on the occasion of "Zabad the Ephraimite, and Shuthelah, etc., whom the men of Gath, who were born in the land, slew; and Ephraim their father mourned many days, and his brethren came to comfort him," 1Chr 7:20-22 : but to what defeat of the Ephraimites this refers is not certainly known; probably the Israelites after the division of the two kingdoms are intended.