Well, it's the most logical explanation. Most of what was written in the New Testament came after he died, and his followers were undoubtedly trying to make sense of it all. After all, he said his kingdom would come to be during their lifetime. But it didn't. So, you either accept that the person you devoted your life to was killed before he could accomplish his goal, or you posit that in some mystical way, he succeeded and his goal is going to be accomplished... very soon. One way you still have hope and you don't feel like your life was wasted and his was destroyed and nullified. The other... well... depressing.
Which parts were written before?