  We present a detailed analysis of the physical processes that cause halo
assembly bias -- the dependence of halo clustering on proxies of halo formation
time. We focus on the origin of assembly bias in the mass range corresponding
to the hosts of typical galaxies and use halo concentration as our chief proxy
of halo formation time. We also repeat our key analyses across a broad range of
halo masses and for alternative formation time definitions. We show that
splashback subhaloes are responsible for two thirds of the assembly bias
signal, but do not account for the entire effect. After splashback subhaloes
have been removed, we find that the remaining assembly bias signal is due to a
relatively small fraction ($\lesssim 10\%$) of haloes in dense regions. We test
a number of additional physical processes thought to contribute to assembly
bias and demonstrate that the two key processes are the slowing of mass growth
by large-scale tidal fields and by the high velocities of ambient matter in
sheets and filaments. We also rule out several other proposed physical causes
of halo assembly bias. Based on our results, we argue that there are three
processes that contribute to assembly bias of low-mass halos: large-scale tidal
fields, gravitational heating due to the collapse of large-scale structures,
and splashback subhaloes located outside the virial radius.
