Say Jack loans Jeff some film that can be destroyed by light. Mark trespasses on Jeff's property and exposes the film to light. Jack can sue Mark for the damages he suffered as a result of Mark's trespass. If Mark wasn't trespassing, Jeff would be the primary responsible party, because he certainly can reasonably be expected to protect film from light exposure by those he authorized. But he can't police an intruder. (You might argue Jack should sue Jeff and then Jeff has to add Mark as an additional party. But if Jeff ignores the lawsuit, Jack should certainly be able to go after Mark directly.) If a wrong against one party harms another, the harmed party has a suit.

Now imagine Jack loans Jeff his poem pursuant to a contract where Jeff agrees not to share it. If Mark conspires with Jeff to share the poem even though Mark promised not to let anyone do that, the contract is enforceable against him under a direct agency theory. If he doesn't conspire with Jeff, he has to trespasses to get the poem, Jack has an action against him because his trespass makes him an indirect party just as in the film example.