In the one case I'm aware of, it was a combination of:
1. Clinical deficiency - i.e., knowledge base not up to par with PGY level. A senior resident should know not to place a VAC on an actively-infected wound, for example. Poor inservice scores can also fit in here, although not as big a deal as recurrent knowledge-based issues with patient care.
...plus....
2. Communication problems - poor sign-outs, failure to obtain coverage for call shifts when you had an interview, unable to be reached by phone or pager when on home call
...with a finishing touch of...
3. personal professionalism issues - unable to take constructive criticism, constantly deflecting blame to co-residents, fighting with nurses over minutiae, inability to be a team player
I can think of many, many residents who filled one of each of these categories, but it took all three to be placed on probation. Which is why, when folks say they were placed on probation, I usually assume there was something fairly significant going on.