It's out of character for the boy Jeremy,who admits to things going wrong around 1978,though he doesn't elucidate.
Think he had a few difficult years around the age of 17/18. well he's not alone in that, most teenagers do suffer quite a lot, argue with their parents and generally aren't quite sure who they are.
Should think Jeremy's time away from the family travelling and having independent experiences helped him to get things back into place which was why he settled down when he eventually returned and was much more involved with the farm.
There's nothing 'strange' about it Steve, it's natural development and maturing.
