Crosstalk between transmission lines

In the case of capacitive coupling, the mechanism is the change in an electric field perpendicular to the conductors, so there's no reason the resulting current would not flow in both directions.

In the case of inductive coupling, the mechanism is the change in a magnetic field wrapping around the conductors. The direction of wrapping of a magnetic field depends on the source current via the right hand rule, and the direction of current induced by a change in a magnetic field is forced by Lenz's law to be in the direction opposing the change in field.

Or to put another way, because when you apply Maxwell's equations to the situation and reduce, that's what the math says.

It might help if you thought of the first situation as if there were a tiny off-the-shelf capacitor between the lines. And the second as if they were wound through a small transformer core.