Dumrod knows that his grip over these dwarves could be broken anytime. There are a lot of factors that could make them break his control even after they make a pledge to him. A real dwarf pledge comes not from being subdued but from their respect.
This is also the reason why Elder Lotord didn't stop Dumrod. He knows Dumrod will never be able to control and become the dwarf king continuously, especially for strong dwarves. The dwarf race has an innate talent for being stubborn as a rock.
This stubbornness sometimes could lead to many magical things, such as breaking powerful mind control or even magical restrictive contracts. Therefore, Dumrod is not panicked and surprised at Forgus's strong reaction this time.
