its quite possible that the WU you had was full of interference on the radio signal, usually from a manmade source.. the seti client will read the WU and if it decides that there is no worthwhile data in the WU it will complete the WU, write the result file and send it straight back.... it doesn't happen very often.. The_one had a work unit of 1 second once.. we both watched in awe as the machine dialed up, got the WU, disconnected, then immeidately dialed up again,.. hehehe
thats probably the likely reason for it..