It could also be that they were simply difficult to work with. Not because they were lazy or incompetent, but just because their style didn't fit with the team.

For instance, somebody who is only productive in absolute silence and prefers working from 6am to 2pm, will have a very difficult time working with somebody who needs to listen to heavy metal at full blast and likes working from 1pm to 9pm.

Especially if they have to work on the same team and need to closely collaborate to get their work done.