  The glassy state is known to undergo slow structural relaxation, where the
system progressively explores lower free-energy minima which are either
amorphous (ageing) or crystalline (devitrification). Recently, there is growing
interest in the unusual intermittent collective displacements of a large number
of particles known as "avalanches". However, their structural origin and
dynamics are yet to be fully addressed. Here, we study hard-sphere glasses
which either crystallise or age depending on the degree of size polydispersity,
and show that a small number of particles are thermodynamically driven to
rearrange in regions of low density and bond orientational order. This causes a
transient loss of mechanical equilibrium which facilitates a large cascade of
motion. Combined with previously identified phenomenology, we have a complete
kinetic pathway for structural change which is common to both ageing and
crystallisation. Furthermore, this suggests that transient force balance is
what distinguishes glasses from supercooled liquids.
