  We investigate the valley relaxation due to intervalley coupling in a
single-electron bilayer graphene quantum dot. The valley relaxation is assisted
by both the emission of acoustic phonons via the deformation potential and
bond-length change mechanisms and $1/f$ charge noise. In the perpendicular
magnetic-field dependence of the valley relaxation time $T_1$, we predict a
monotonic decrease of $T_1$ at higher fields due to electron-phonon coupling,
which is in good agreement with recent experiments by Banszerus et al. We find
that the dominant valley relaxation channel in the high-field regime is the
electron-phonon coupling via the deformation potential. At lower fields, we
predict that a peak in $T_1$ can arise from the competition between $1/f$
charge noise and electron-phonon scattering due to bond-length change. We also
find that the interlayer hopping $\gamma_3$ opens a valley relaxation channel
for electric charge noise for rotationally symmetric quantum dots in bilayer
graphene.
