Does direction of current flow affect induced current?
It may be inductance but things like this are hardly ever that simple. If the spikes you are seeing are at the turn on and turn off edges of the PWM signal my bet is that it DOES have a capacitance coupling component. Just because trace capacitance is small doesn't mean it won't couple. It is the relative capacitance trace to trace and trace to other low impedance (be it ground or a power rail).

Ceramic capacitors are usually good for taking care of noise issues like these. The trick is where to put them. Make sure to have some on IC rails and close to switching circuit where high dI/dt signals reside.

Also make sure that the scope probe ground you are using is small and very near the signal you are measuring. The signal you are seeing could be radiating into the scope probe and fooling you.