It happens when there's an area of low pressure well north of the area, and an area of low pressure well south of the area.

Since air wants to head toward lower pressure, you have some air being pulled north, and some being pulled south. The void left behind creates a vacuum of sorts, pulling up air from near the ground to replace it.

As that air rises, it condenses, bringing clouds and rain. Deformation bands tend to bring a steady, yet light-to-moderate rain.

It can bring rain for a good 6-18 hours straight, even with no organized rain systems around for hundreds of miles.