Standing in for the king is a job that has to be done the king's way, or not at all. Shebna did it his way, not the king's, and Shebna's time as the king's second in command is over. So how does Shebna's moment of fail relate to the other readings in Ordinary Time?
In Jesus' parables, whenever a king or ruler says or does something, Jesus is using that character to tell us what God would do or say about what is happening in the story. The Old Testament reading serves the same purpose, only there is no character standing in for God. God tells us directly how he sees things.
So why is this placed in with all of the stuff about the kingdom of heaven?
To let us know that while God's invitation is open to all, it is open on God's terms. If I don't want to accept God's invitation God's way, and I insist on trying to do things my way, then I should not be surprised if I see others entering in - and not me.
More on Sunday's readings tomorrow.