The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded ~1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011–2015) of 0.58% yr$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, but large spatial variations are present. Using the nonparametric "Demons" method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from $0.\prime \prime 09$ to $0.\prime \prime 44$ yr−1. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities v s as low as 3600 km s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ (for an assumed distance of 8.5 kpc), much less than v = 12,000–13,000 km s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at $1.3 % \pm 0.8 % $ yr$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion.