Rashi #1: It is like one who oversteps his friend's border. So they rushed to adopt the ways of Malchei Yisrael, their fellow kings.

Rashi #2: They stole others' fields ¹ . Radak ? this is like "Arur Masig Gevul Re'ehu" (Devarim 27:18). Officers of Yehudah moved the border markers of their weak neighbors, and took from their inheritances. So it says in Yechezkel "v'Lo Yonu Od Nesi'ai Es Ami Lehonosam me'Achuzasam" (45:8).

Radak citing Rav Sadya Gaon: They are those who encroach the borders of Mitzvos. Sarei Yehudah were like Sarei Efrayim, who transgressed Alos ha'Bris.

Radak citing his father: The officers were like those who steal others' fields. If Dasan moved his border to steal land from his neighbor Kalev, and Kalev screamed to the officers, the officers did not heed him. Via supporting the thieves, it is as if they moved the border.

Radak citing Ibn Ezra: They did evil as much as they could, like those who move the border covertly.

Malbim: "Shivtei Yisrael" (9, the 10 tribes), now they are Sarei Yehudah, i.e. they rule over Yehudah. It is as if they overstepped the border of Malchus Beis David. Hashem promised David that his seed will always rule over [at least] one Shevet, like I explained in Melachim I, Perek 9.

Malbim: "Hodati Ne'emanah" (9) is a faithful, decided decree that "Eshpoch ka'Mayim Evrasi" on them ? "uv'Od Shishim v'Chamesh Shanah Yechas Efrayim me'Am" (Yeshayah 7:8). Evrah is punishing amidst anger, and also people who did not sin are punished. The primary sinner was Shevet Efrayim, i.e. Beis ha'Malchus, which made the calves. Shivtei Yisrael (the other Shevatim) were enticed after them; it is as if they were forced and Mefutim ¹ . Even so, the anger against Efrayim will become an Evrah of the Klal to punish also the other Shevatim, like spilled water spreads in every side.