If someone told me that a dead person they knew had come back to life, I think my first question would be "How do you know that he was dead?" If they didn't have any credible evidence, I think I would be justified is suspecting that the guy had never really died.
Our evidence for Jesus's crucifixion consists of stories passed down by men who claimed to have seen him alive after he had been crucified. For the most part, however, these stories give us little reason to think that the men were likely to have seen Jesus crucified. As his followers, they had every reason to be in hiding out of fear that they might be arrested and crucified along side Jesus. So if the men who claim to have seen Jesus returned from the dead mightn't have been in any position to verify his crucifixion and there is no independent corroboration for the event, mightn't that be sufficient reason for a historian to doubt that it actually took place?
Perhaps when the crowd of men with clubs and swords showed up at the Garden of Gethsemane, Jesus and his disciples scattered and most of them got away. Judas, wishing to collect his thirty pieces of silver, simply kissed one of the men who had not gotten away and that man was crucified. Jesus managed to make his way back to Galilee while his disciples, assuming that he had been crucified, remained in hiding in Jerusalem. Later when they encountered Jesus in Galilee, they concluded that he had returned from the dead and not wanting the Romans to find out that he hadn't been crucified, Jesus declined to disabuse them of that notion. I cannot see how that is any more far fetched than the idea that he actually returned from the dead.
I think that the simplest explanation for a claim that a person has been encountered alive is not only that he is not dead, but that he has never been dead. Before embracing either hallucination or resurrection as the explanation for the encounter, I think a historian would wish to corroborate that the person had in fact died prior to the claimed encounter. It is not hard to imagine a scenario in which Jesus's followers might mistakenly believe that he had been captured and crucified.