Orcs are brutish creatures , but they are not wholly unintelligent.
Clever enough to stay still, play dead. Clever enough to jump up when the opportunity arises.

Elrond is distracted by the pale counternance of his silver queen, her silence. He hears the Orc snarl as it hurls itself at them and twists his body to shield her, as he reaches for the long knife in his belt.

Leaving his side unguarded.