Ramban himself explains why he feels it is acceptable to argue against Chazal (the Sages of the Mishnah and Gemara) when explaining the simple meaning (p'shat) of a verse in the Torah.
In his comments to Bereishit 8:4, he writes that is permissible for him to explain some of the details of the flood story differently to Rashi, whose explanation there follows the approach of Bereishit Rabbah.
The key line is:
כיון שרש"י מדקדק במקומות אחרי מדרשי ההגדות וטורח לבאר פשטי המקרא הרשה אותנו לעשות כן כי שבעים פנים לתורה ומדרשים רבים חלוקים בדברי החכמים
Since Rashi often asks pointed questions against aggadic midrashim and troubles himself to explain the simple meaning of the text, he has allowed us to do the same [here]. For there are seventy faces to the Torah, and many midrashim that present arguments within the words of the Sages.
So we see that Ramban believes that it is perfectly acceptable to present a different p'shat-approach, which is in opposition to an explanation of Chazal. There are a large number of possible, acceptable interpretations of a verse. Chazal picking one of them does not preclude him from preferring another.
Thus, it should not be surprising that Ramban is willing to explain the sin of the Golden Calf somewhat differently to the way it was explained by Chazal (and Rashi).
