Why were Rosencrantz and Guildenstern sent for by King Claudius and Queen Gertrude?

Rosencrantz and Guildenstern were sent for by the King and queen to keep an eye on Hamlet to figure out why he was acting strangely.

Explanation:

The king and queen were greatly disturbed by Hamlet's erratic behavior and the difficulty he was having recovering from his father's death. They summoned Rosenberg and Guildenstern with the hope that they be able to cheer Hamlet up or at least get to know the reason for his behavior. The friends agree to do this and Gertrude ordered attendants to take them to the much changed Hamlet.