Echoing a CSS class based on category of post in a list

get_category('91') is always going to return an object of category ID 91’s data. This means your if statement is always going to return as true. You need to get the current iterated posts assigned category ID and then check if it is 91.

<!-- Return objects of current iterated posts associated categories -->
$categories = get_the_category();
<!-- Get current iterated posts assigned category ID from $categories object. The [0] is because a post can be assigned to multiple categories. 
In your example you are probably only assigning one category per post, so you just need to grab the ID of the first category in the returned object. -->
$category_id = $categories[0]->cat_ID;
<!-- Check if current iterated assigned posts category ID is equal to 91 -->
<div class="bfa-category <?php if ($category_id == 91) echo 'bfa-cat-s1'; ?>">
    <?php the_category(); ?>
</div>