I am not sure how relevant my personal experiences are here. Three of my tours were with others at least to start.
Two trips (full TA with my daughter and a friend, and a 30 day tour in the Sierras with my daughter) we stayed together the whole time with a little time where we rode alone for part of a day every now and then. The other trip, my online friend who I met in person only for this tour, fell and fractured his pelvis maybe 500 yards into the tour. The third guy on the trip immediately bailed when his friend was injured.
That said I met a lot of folks on the Trans America who were fragments of a split up tour. Most of them had little to say about why they split up. Mostly it was just day to day friction between them as far as I could tell. The exceptions were where someone was injured, these were significantly less common though.