What is ironic about the King sending for Rosencrantz and Guildenstern?

The arrival of Hamlet's friends is supposed to take him off guard..... please him. Their presence is supposed to make Hamlet open up..... to cheer him up. Ironically, their presence irritates him, and it takes no time whatsoever to make them admit they came not so much out of friendship, but at Caludius' bidding.

Why might the king be worried about hamlet behavior