Hi @pherocactus
This confused me at first, but is probably just the result of using the Transpose tool to move two meshes that are too close together apart while X symmetry is active. Transpose operations can produce odd results when scaling or moving along axes for which symmetry is also active. It basically creates two opposing versions of the transpose operation, and sometimes these operations can overlap in unexpected ways.
You’ll notice that you can move them together without issue, but once you start to move them apart again, the meshes are now too close to produce good results with symmetry. If you did not move the mesh together first, you could probably move them apart without issue.
You can also disable symmetry, move only one mesh, then use Tool > Geometry > Modify Topology > Mirror and Weld to mirror the boot to the other side. This only works in one direction, so you may need to move one boot and not the other to get it to mirror correctly.