I had same problem. I think boinc client resets the project himself if he encounters errors in data files or mismatches in client-server communication state. In my case, the reason was a boinc data directory located on network share and death of network switch. A boinc data directory restored from backup could cause similar behavior.
Although all slots, checkpoints and control files were in place, when restarted client contacted the server, it marked all workunits as abandoned. I returned few but not sure if I get any credit for them, and their status on server wasn't changed.