We apply a tension metric $Q_textrm{UDM}$, the update difference in mean parameters, to understand the source of the difference in the measured Hubble constant $H_0$ inferred with cosmic microwave background lensing measurements from the Planck satellite ($H_0=67.9^{+1.1}_{-1.3}, mathrm{km/s/Mpc}$) and from the South Pole Telescope ($H_0=72.0^{+2.1}_{-2.5}, mathrm{km/s/Mpc}$) when both are combined with baryon acoustic oscillation (BAO) measurements with priors on the baryon density (BBN). $Q_textrm{UDM}$ isolates the relevant parameter directions for tension or concordance where the two data sets are both informative, and aids in the identification of subsets of data that source the observed tension. With $Q_textrm{UDM}$, we uncover that the difference in $H_0$ is driven by the tension between Planck lensing and BAO+BBN, at probability-to-exceed of 6.6%. Most of this mild tension comes from the galaxy BAO measurements parallel to the line of sight. The redshift dependence of the parallel BAOs pulls both the matter density $Omega_m$ and $H_0$ high in $Lambda$CDM, but these parameter anomalies are usually hidden when the BAO measurements are combined with other cosmological data sets with much stronger $Omega_m$ constraints.
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