One reason could be UE has lost radio connection with source eNodeB. In that case, there is no need to complete rest of the handover signalling messages since source eNodeB would not be able to send handover command message to UE even though target prepares radio resources. This is a variant of one of the too late handover cases.
Another case, UE went into inactivity mode in which UE does not hold radio connection with the eNodeB.
