There aren't any settings to change, as the default ratio is the image's actual AR unless you've previously used a Pan & Zoom effect. At that point the project's AR is applied.

For example, if I place one of the 16:9 sample images in a 16:9 project then click Tools > Pan & Zoom, no matter which style I use, the Motion Designer opens with 16:9 selected.

If I change the project's AR to 4:3, doing the same steps and selecting any style except User Defined results in the clip being cropped to 4:3 and the Motion Designer opens with 4:3 selected. The only time that doesn't happen is if I choose User Defined the first time when working with this clip, and then the clip retains its 16:9 AR and the MD's AR matches that.

You could be seeing Freeform selected if you're working with clips with a non-PD-standard AR, like 21:9 or 5:4. Is that the case? Also, once you've applied any keyframes in Freeform mode, MD should open with that AR selected. If you've previously applioed any 16:9 keyframes, opening that clip in the MD again should always have 16:9 selected.

Although there is a Reset option in the Pan & Zoom window, it may be better to delete the timeline clip and drag a fresh copy to the timeline from the media library if the MD doesn't automatically choose the correct AR setting after you've made any previous Pan & Zoom edits.