Basically, crosswise grain shrinks more for the exact same reason that it stretches more. Crosswise grain (the weft threads) will always shrink and stretch more than the lengthwise grain (the warp threads), but it's due to the weaving process, not the printing. In weaving the fabric, the warp threads are stronger and are held tightly in place on the loom. The weft threads are then run back and forth over and under the warp.