I would assume it's a statement on determinism. The phrase is based on the dual usage of "just" in meaning "simply" the first time, and then "rationally" in the next. It seems the author wants you to refuse the simple understanding of things and focus on the causal chain-of-events leading up to them instead.
That's just my interpretation, though. "Just" is also commonly used as a term for religious righteousness, which could also change the meaning. I'm unfamiliar with Jim Rohn too, if that helps :P
I guess the one saying about things happening for a reason, or going further, the reason why they happen is to give us a chance to act on them in some way, talks to me the most.
You're right to be confused, it's fairly ungrammatical, one would typically use the adjectival form there: "... things happen justly" but then it wouldn't scan as well.
