Was it not proper to rejoice that the Aron returned? Why were they punished (verse 19) for being happy to see it?
Rashi (from Tana d'Vei Eliyahu Rabah 11): They looked amidst light-headedness, without fear and honor.
Radak (from Bamidbar Rabah 8:9): Amidst Simchah, they opened it to see what is inside; it says "v'Lo Yavo'u Lir'os... va'Mesu" (Bamidbar 4:20). This is the best Perush; refer to 6:19:1:1.
Radak (19, citing Sotah 35a) #1: They bowed to it without pausing from reaping.. Malbim - they did not have the proper Simchah of Mitzvah to cease working and go out to greet it with great honor and glee. They merely rejoiced like one who sees a new matter that astounds its seers.
Radak (19, citing Sotah 35a) #1: They spoke disrespectfully - 'who angered you (that you allowed yourself to be captured), and who appeased you (that you returned on your own)?' 1
Malbim: Refer to 6:19:1:2.
Radak (19): According to this, Ra'u (19, and perhaps also va'Yir'u in our verse - PF) is like "Lera'avah Vach" (Yechezkel 28:17), i.e. disgrace.
