My best guess is, they collected campfire tales about different messiah type figures roaming the land. There was an abundance of self declared prophets in Palestine at the time. The priesthood had been appointed by the Roman authorities, was entirely dependent on them and wasn't exactly popular with the ordinary people. So everyone preaching against them could find their audience pretty easily. One or more of these figures might have even been crucified. But rather for something the Romans didn't like than according to the wishes of the string puppet jewish priesthood.
And as it goes, the tales have been embellished with myths that must have been pretty well known to the original writers, since they all apply to Greek and Roman deities.
And as it goes, the tales have been embellished with myths that must have been pretty well known to the original writers, since they all apply to Greek and Roman deities.