  In the quest for materials with unconventional quantum phases, the organic
triangular-lattice antiferromagnet $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ has been
extensively discussed as a quantum spin liquid (QSL) candidate. Recently, an
intriguing quantum critical behaviour was suggested from low-temperature
magnetic torque experiments. Through microscopic analysis of all anisotropic
contributions, including Dzyaloshinskii-Moriya and multi-spin scalar chiral
interactions, we highlight significant deviations of the experimental
observations from a quantum critical scenario. Instead, we show that
disorder-induced spin defects provide a comprehensive explanation of the
low-temperature properties. These spins are attributed to valence bond defects
that emerge spontaneously as the QSL enters a valence bond glass phase at low
temperature. This theoretical treatment is applicable to a general class of
frustrated magnetic systems and has important implications for the
interpretation of magnetic torque, nuclear magnetic resonance, thermal
transport and thermodynamic experiments.
