both males and females will pit in anticipation of breeding. sometimes it's the breeding pair together (usually requires some past experience), sometimes a second (or third) bull will pit in other areas to compete for that female, sometimes a valid (tube down) female will pit alone (because no male is acceptable to her terms today), and sometimes a second ( or third) female will pit because, after all, they are harem breeders (this is a bit rare, usually a male will finish with one female, before joining another. so it can be common to see 3-4 females holding in a weeks time. just not all of them on the same morning). and last, but not least, sometimes any front will pit because it senses something edible underneath.
with a bit more detail shared here, or careful observation of the entire group by yourself over the next few days, the right answer will become more evident. HTH.