While Grace is teaching the Eridians, he mentions how important socialization and pack bonding is for humans. Humans require a community to thrive and get depressed if they're left alone, so it's best to keep them in groups. However humans can pack bond with anything, and Grace is very close to the Eridians, so he's okay.
The Eridians misunderstand him, and proceed to panic because they only have one human. They only have one human but it turns out you need to keep them in packs. They can die if they're isolated for to long and they can't let Grace die this is bad bad bad.
So they send a message to earth, asking if anybody would be willing to volunteer to live on Erid with Grace. They stress that while this is a life or death matter, they need the human to be a willing participant. That all of their needs and wants would be met and that the Eridians would give Earth information and technology in exchange. The volunteer would be given a life of luxury in exchange for living on Erid with Grace.
Unfortunately, there is a mistranslation, and the humans think that the Eridians require a sacrifice otherwise they will use their resources against earth. In order to avoid a possible interstellar war with an alien species, they choose a convict who has a life sentence to be locked in a bare rocket and blasted off to Erid.
That is how Simon, the only survivor of his religious extremist cult, ended up on the SM-13 headed to an alien planet against his will.
