Why does the Torah write "Es Kutonto, Es Kesones ha'Pasim Asher Alav"?
Rashi: Because they stripped him of the two robes that he was wearing, the one that he wore regularly and the one that his father made specially for him. 1
Mizrachi: He was wearing only one robe - and "Es Kesones ha'Pasim Asher Alav" is coming to explain "Es Kutonto." Ha'Emek Davar - you cannot say that they cast him in naked, and he was sold naked!
Oznayim la'Torah: Yosef was wearing two robes and that they stripped him only of the Kesones ha'Pasim. See his proof.
Moshav Zekenim, Hadar Zekenim, Riva: Presumably, the special one was on top, in which case, they should have removed it first! They removed from the bottom (the regular robe was longer). Alternatively, travelers normally put a [simple] robe over their [dignified] clothes to cover them - Moshav Zekenim. Ohr ha'Chayim - they intended only to remove his special robe on top, which they envied. In their fury, they were not careful to remove only it, and they clutched also his inner robe with it, and threw him into the pit naked.
Why did they strip him of his special robe?
Rashbam: In order to dip it in goat's blood, to mislead their father into believing that a wild beast had killed him. 1
Ohr ha'Chayim, Malbim: It was this one that they envied. Ha'Emek Davar - also, they wanted to bring it to their father (verse 32). And even if it were the same as their robes, when a distinguished person sins and must be punished, first we remove his distinguished robe (a sign of his importance), and then we lash him (Yerushalmi Horiyos 3:1).