  We revisit the localization computation of the expectation values of 't Hooft
operators in $\mathcal{N}=2^*$ SU(N) theory on $\mathbb{R}^3 \times S^1$. We
show that the part of the answer arising from "monopole bubbling" on
$\mathbb{R}^3$ can be understood as an equivariant integral over a
Kronheimer-Nakajima moduli space of instantons on an orbifold of
$\mathbb{C}^2$. It can also be described as a Witten index of a certain
supersymmetric quiver quantum mechanics with $\mathcal{N}=(4,4)$ supersymmetry.
The map between the defect data and the quiver quantum mechanics is worked out
for all values of N. For the SU(2) theory, we compute several examples of these
line defect expectation values using the Witten index formula and confirm that
the expressions agree with the formula derived by Okuda, Ito and Taki. In
addition, we present a Type IIB construction -- involving D1-D3-NS5-branes --
for monopole bubbling in $\mathcal{N}=2^*$ SU(N) SYM and demonstrate how the
quiver quantum mechanics arises in this brane picture.
