The pattern is that those fired had their workloads doubled or tripled and then told that they were not being productive enough. This was to force them to quit, or else they terminate them for some reason. No evidence needed, no defending themselves, no appeal. It didn't matter if they did what they were accused of, as long as management believed that they did it, without any hard evidence, they got let go.
Any hard evidence of this is at the firm, unless they destroyed it. But in order to prove it we would have to go into the corp email system, the project management database, and HR records. Which may be destroyed after an investigation starts.
The firm got investegated by the FBI for fraud a few years ago, and they blamed it all on a partner, and any evidence he needed to prove his innocence mysteriously vanished. If they can do that to their own, imagine what they can do to an employee or ex-employee?