Because they refused to convert to Islam:

Some of the South Korean Christian aid workers held hostage by Afghanistan’s Taliban said they were beaten for refusing to convert to Islam and protecting female captives, a hospital chief said Monday.

“We found through medical checks that some male hostages were beaten,” Cha Seung-Gyun told reporters after the 19 freed aid workers — 14 women and five men — underwent examinations at a hospital outside Seoul.

They had returned home Sunday after six weeks in captivity.

“They said they were beaten at first for refusing to take part in Islamic prayers or for rejecting a demand to convert,” Cha said.

(via)