particles that go into marble (to my knowledge) don't have any noticeable elongation
Not necessarily. Marble is mostly made of either calcite (CaCO3) or dolomite (CaMg(CO3)2), both of which are carbonates that do not exhibit any foliation in metamorphic rocks. But marble is rarely pure and commonly has some clays or other minerals that can contribute to any linear features. Notice that the stuff in your picture is quite big - you can have thin layers of not-pure-marble between the pure marble.
does it have something to do with the magma intrusions that contribute
to the marble formation process
Marble forms by several processes, not always related to a magma intrusion. Marble forms by heating of carbonate rocks. It could be magma and it could be other stuff.
That said, marble can definitely have veining. More commonly the veining is the result of hydrothermal fluids, that is mostly supercritical water flowing through the rock (in veins), dissolving some minerals and precipitating others.
Sometimes you can have textures like that even without any minerals that can be foliated at all. If you take interbeds of calcite and black sand (quartz + heavy minerals, let's say magnetite) and start deforming it, you will get interesting textures. Go to Google Images and type in "metamorphic deformation".