I was just thinking about this guy today. We would have been in similar crowds at that time, and Eric Hager & I have mutual, real life friends, although I did not know either of them at all. (Just to be clear.)
Here's what I think happened:
Grateful Doe went to college & had a good time. I think his family discovered that he was getting high and flipped out, telling him they were done with him. I have friends from that era whose parents did exactly that, over pot, and have never spoken w/ their now 30- or 40-something year old kids since. Seems ridiculous, but not rare in the circles I ran with.
I think he was without a backpack b/c he left his bag in an acquaintance's car. Back then, we had Ride Share boards on campus, and it was very common to grab a ride w/ a total stranger from your college if they were headed to the same show. It wasn't like going w/ your buddies. Just riding along & splitting the gas, but not "responsible" for each other once at the show, like a friend would be. If you missed the ride back, tough luck, and nobody thought anything of it & figured you stayed around longer or got another ride back.
I think GD was in school (college) somewhere in the South, and a ride in that direction got him closer, even if not exactly where he wanted to go.
I would not have considered this "hitch hiking" at the time b/c it was these loose connections of friends and acquaintances, sort of like these days, if you met up w/ a Facebook "friend" that you don't actually know.
If he was considering dropping out of school, his roommate(s) might not even have thought anything of him not returning, especially if he used drugs and they did not. They might have "looked down on" him and not thought anything of it, esp if he was a freshman, where roommates are often thrown together, not selected.
I hated my freshman roommate & didn't even know where she hung out most of the time & certainly would not have known if she went away for a weekend where she was or who she was with. Just my $0.02 from "college life" at that time.