Wo ist der Bu… die B… fein, wo ist das gelbe Ding?

Jokes aside, it’s common in gendered languages to have a handful of nouns with a variable gender. In this case, it was likely caused by regularisation; the word is originally feminine but it looks masculine, so eventually you got people using the masculine with it.

(I think that der Butter is specially common in Ba-Wü and Switzerland, but I’m not too sure.)

For reference, examples of the same happening in other languages:

• Catalan - el mar (masculine) vs. la mar (feminine). Both mean “the sea”. I think that “el mar” is due to Castilian interference, given that Castilian uses primarily the masculine while Occitan uses primarily the feminine.
• Portuguese - o omelete (masculine) vs. a omelete (feminine). Both refer to omelet, frittata etc. The masculine is more common but it makes pedants scream bloody murder.