I think the strong presumption of "must" comes from its obligation nuance. It's like such obligation should be applied to strong presumption.
It's like "I strongly guess he is married, so such presumption will bring about his obligation of being married", what do you think?
1. He must get married. - obligation
2. He must be married. - strong presumption
Probably this is too hard to answer - if even I think about some reason why words work this way or that in Korean, I wouldn't know it exactly.