First of all, you are confusing two verbs: ser and estar. Fueron is the perfect tense of ser and the imperfect conjugation of that would be eran, not estaban. So your question should be ''do I use fueron/eran'' instead, because fueron/estaban is mixing and matching two different verbs. Now, in this context the verb estar makes no sense, as this verb is generally used to describe locations and conditions. When you say a third of the bull-related festivites were the bull runs you have to use a conjugation of the verb ser in Spanish, not estar, so right off the bat we can discount estaban as that makes no grammatical sense in this case.
I don't often trust online translators such as google translate but in this case it is correct, the required form is indeed fueron. It's not as simple as always using the imperfect for a repeated action in the past. In this sentence you are talking about 2021, so in this context it functions as one precise moment in the past and therefore it must take fueron and not eran.