Red is generally a final ripening stage for most peppers, indicating they are fully mature. There are many different kinds of mild chilis, however, referred to as "banana peppers" and some of them eventually become red instead of golden yellow. It is also definitely possible you have something other than a true banana pepper growing - through mislabeling or cross-pollination of the seed used to produce the plant.
I'd go by taste rather than by appearance in deciding which is the most likely case - unless these are "Hungarian" Banana Peppers, they should be mild, slightly sweet, and slightly tangy in taste. If they are hot, then you definitely have something that's not your regular banana pepper.