Ok so this is a crude drawing but it gets the point across
In the top picture, A, the axle is level and both the drag link and tie rod extend to the end of the axle tubes, leaving both tires pointing straight.
In B, the Passengers side is compressed and the drag link extends to the end of the tube leaving the passenger tire straight, but the tie rod no longer reaches the end of the axle tube which will cause the drivers knuckle to turn in towards the center of the vehicle, and then of course points the drivers tire inward.
While it's more of a situation where the toe is no longer correct, it does create the bump steer problem.
In a stock style steering set up it is nearly impossible to have a drag link and track bar that are the same length since the drag link and track bar are relatively close where they mount on the frame end, but the drag link is significantly longer having to extend all the way out to the wheel.
While I didn't illustrate crossover steering, it's easy to see from this example that if the tie rod extends from knuckle to knuckle, then the relationship of the tires to each other remains constant, regardless of the position of the axle.
Bump steer can still occur in crossover steering if the track bar and drag link aren't the same length and aren't on the same angle. This happens because as the passenger side swings upward, it causes the tire to want to naturally turn towards the passenger side as the axles changes it's lateral position relative to the body. However, in crossover, it doesn't cause a change of toe but will cause both tires to follow the same path when the bump steer occurs, so if one turns to the right the other does as well, and they don't turns towards each other. If both the track bar and drag link are the same length, the axle tends not to change it's lateral relationship to the body and the drag link follows the same path track bar, which removes the bump steer, and allows the tires to remain pointed straight as the axle compresses on one side or the other.
My apologies if this seems all run together, I'm working from memory.