וַיַּרְא֙ חֹתֵ֣ן מֹשֶׁ֔ה אֵ֛ת כָּל־אֲשֶׁר־ה֥וּא עֹשֶׂ֖ה לָעָ֑ם וַיֹּ֗אמֶר מָֽה־הַדָּבָ֤ר הַזֶּה֙ אֲשֶׁ֨ר אַתָּ֤ה עֹשֶׂה֙ לָעָ֔ם מַדּ֗וּעַ אַתָּ֤ה יוֹשֵׁב֙ לְבַדֶּ֔ךָ וְכָל־הָעָ֛ם נִצָּ֥ב עָלֶ֖יךָ מִן־בֹּ֥קֶר עַד־עָֽרֶב׃
Y viendo el suegro de Moisés todo lo que él hacía con el pueblo, dijo: ¿Qué es esto que haces tú con el pueblo? ¿por qué te sientas tú solo, y todo el pueblo está delante de ti desde la mañana hasta la tarde?
Sforno on Exodus
מדוע אתה יושב לבדך?, why are only you dealing with public affairs?
Rashbam on Exodus
מדוע אתה יושב לבדך?, and as a result of your being the sole judge the entire people have to stand in line before you from morning to evening.
Mekhilta d'Rabbi Yishmael
(Exodus 18:14) "And Moses' father-in-law saw": What did he see? He saw him sitting like a king sitting on his throne and all paying attendance upon him, whereupon he said to him: "What is this that you are doing to the people? Why are you sitting alone?"