Crosstalk waveforms are a function of the direction of current flow. For example, in Figure 1, if we reverse the direction of current flow on the aggressive trace, the crosstalk received on the victim will reverse polarity. First, set up the circuit as in Figure 1. Next, reverse the direction of current flow on the aggressive signal. The polarity reversal tells us that we are not dealing with capacitive crosstalk.
Many digital engineers assume that crosstalk is primarily a capacitive effect.
Mutual capacitance between two single-ended circuits can only cause positive crosstalk. That's the same kind of coupling you get in a transformer. It is well-known that reversing the leads on the primary winding of a transformer will reverse the polarity of the voltage on the secondary. Coupled pc-board traces act in much the same way. The aggressive current makes a loop.