That really is the core of the mystery. You're right that he must have a past, but it sure seems like he did spring into existence on the AT in New York in 2017.
There are so many possibilities. Shame, guilt, fear, depression, desperation. Flight to avoid prosecution. Flight to get away from overwhelming problems. Thrown out by family. What we used to call "midlife crisis." Recovering from grief or trying to find self. Lifestyle mismatch with family.
I could believe a very simple sequence: got laid off from a tech job, decided to hike the ECT since he might never have an unencumbered block of time again, ran into bad luck at the end. But if that was all that was going on, why would he use an alias?
Is Ben Bilemy his real name and we just haven't found it yet?