Rashi #1 and R. Bachye #1 (by implication): He saw that the calf ¹ contained a spirit of life ² and that the Satan was succeeding, so he realized that it was not possible to reject their request completely.

Rashi #2, R. Bachye #2 and Targum Yonasan: He saw that they had murdered his nephew Chur, ³ when he tried to stop them and he was afraid ⁴ that if he followed suit, they would kill him as well.

Rashi #3 and R. Bachye #3: He was afraid that if they would kill him (they would fulfil the Pasuk in Eichah, 2:20 "Im Yehareg ba'Mikdash Hashem Kohen ve'Navi", a sin which would have no remedy ? R. Bachye ? and) be guilty of murder, and it was therefore preferable that he should make for them an Eigel, in which case they wold at least be able to do Teshuvah'.

Rashi #4 and R. Bachye #4: He realized that if he were to withdraw and allow them to build the Mizbe'ach, one person will bring earth, the other, stones, and they will complete it in no time. Better that he continues to build slowly, slowly; meanwhile, Moshe was bound to return.

Ramban and Moshav Zekenim: He saw that they were bent on doing evil, that their minds were hooked on the calf and realized that, if left to their own devices, they would build many Mizb'chos to the calf. ⁵

R. Bachye #5: He figured that merely making a calf constituted only a La?av ? ?Lo Sa?aseh l?cha Pesel? - and was not therefore Moser Nefesh over it.

Da'as Zekenim and Hadar Zekenim: He saw that they had erred, and was apprehensive that upon realizing that the calf would not save them, they would appoint a leader. Therefore, he built a Mizbe'ach to delay them until Moshe came.

Rashi and Ramban #1: In order to put them off for as long as possible, in the hope that in the meantime, Moshe would return. ¹

Ramban #2: In honor of Hashem, so that they should sacrifice on it to Him and not build altars for their idols to their shame.

Rashi: He hoped that in the interim, Moshe's would return with the Luchos and they would celebrate his homecoming with a Yom-Tov, and that they would sacrifice on the Mizbe'ach that he had built, in honor of Hashem.

Seforno: He meant that tomorrow would be a Yom-Tov for Hashem, and that they should not exchange it for another god.

Targum Yonasan: Dejected on account of the rebellion that was taking place, he announced that the next day (when Moshe returned) they would make a Yom-Tov to celebrate the death of the rebels - 'rebels who exchanged the Shechinah for this ox'

Da'as Zekenim and Hadar Zekenim: He meant that they would crown their new leader and make a Yom-Tov in the Name of Hashem and offer Shelamim to Him - like Yisrael did when Sha'ul was crowned. ¹

Moshav Zekenim: He meant that tomorrow, Hashem would bring 'Chaga' (destruction) on the heretics.