  PKS 1510--089 is a bright and active $\gamma$-ray source that showed strong
and complex $\gamma$-ray flares in mid-2015 during which the Major Atmospheric
Gamma Imaging Cherenkov telescopes detected variable very high energy (VHE;
photon energies $>$100 GeV) emission. We present long-term multi-frequency
radio, optical, and $\gamma$-ray light curves of PKS 1510--089 from 2013 to
2018, and results of an analysis of the jet kinematics and linear polarization
using 43 GHz Very Long Baseline Array data observed between late 2015 and
mid-2017. We find that a strong radio flare trails the $\gamma$-ray flares in
2015, showing an optically thick spectrum at the beginning and becoming
optically thin over time. Two laterally separated knots of emission are
observed to emerge from the radio core nearly simultaneously during the
$\gamma$-ray flares. We detect an edge-brightened linear polarization near the
core in the active jet state in 2016, similar to the quiescent jet state in
2008--2013. These observations indicate that the $\gamma$-ray flares may
originate from compression of the knots by a standing shock in the core and the
jet might consist of multiple complex layers showing time-dependent behavior,
rather than of a simple structure of a fast jet spine and a slow jet sheath.
