I've been collaborating on a paper with a junior colleague and a senior colleague, among others. The junior colleague is the lead writer on the paper and has done about 85-90% of the research. Junior is an early-career researcher, senior is about in their 60s.
I've been noticing a very odd trend regarding the interactions between these individuals. The senior colleague seemingly has almost nothing positive to say to the junior colleague. They are hyper-critical of their work, constantly invalidates whatever the colleague has to say, and never provides constructive feedback or responds positively to the junior colleague's suggestions. They never try to be constructive and offer alternative suggestions for what the junior colleague's sees in their data, they just say "no, you are wrong". The closest thing to a positive statement they have made was a backhanded compliment.
The senior colleague has been increasingly trying to monopolize their contributions to the manuscript and force out the lead author's contributions. Notably, the two did not mutually agree to be collaborators but ended up as strange bedfellows due to the actions of a fourth party, the junior didn't like the idea but initially said nothing because they wanted to "be a team player".
Several times the senior colleague has actually pulled unpublished data out of their own lab just to be able to say the junior colleague is wrong. This is data that none of the other collaborators are able to cross-verify due to our labs being located in different countries. Notably, this unpublished data only ever invalidates the junior colleague conclusions, the colleague never says "oh yes, this resembles some of my data". Based on the pattern, I almost wonder if the senior colleague is cherry-picking data from their material to do this. The senior colleague does not have access to the specific material we are working on, they are using data from a closely related biological taxon. When the junior colleague pushed back and said (and showed) they had data to support their original assertion, the senior colleague snapped at them and said they had been in the field longer than the junior colleague has.
The junior colleague is getting pretty distraught about the intensity of the criticism and has actually confided in me they think the senior colleague does not like them because prior to this collaboration the junior colleague disagreed with the senior on one of their research theories and according to the junior colleague the senior took it poorly. Notably, that topic was unrelated to the one we are currently working on. The junior colleague is a fairly diligent worker and while they could be wrong with some of their conclusions the extent of the senior author's criticism would imply almost all of the junior author's conclusions are wrong. In a couple of cases the junior author seems to have been right, in some the senior, but I have been unable to evaluate all the criticisms back and forth due to volume.
I have been debating whether it is worth getting involved and saying something. I've been sitting on the fence about this because criticism is a key part of the scientific process and it's entirely possible the junior author could be wrong with their hypotheses. However, it's the broader consistency of these hyper-critical responses and the fact that the senior colleague only ever seems to criticize the junior makes me suspicious if these criticisms are coming from a place of good faith. For the most part, the senior colleague has technically never said anything wrong but the broader pattern is concerning.
At what point would behavior like this cross the line from legitimate scientific discourse and "the data says what the data says" to academic bullying or harrassment that someone needs to step in on?