LR automatically crops my images upon import

To test if the issue is still present with the previously imported files select one of them, hit the Delete key, and select 'Remove from Lightroom.' Then reimport that file and see if the 16:9 crop is still being applied. If so the only explanation is that you changed the camera's setting for 'Aspect Ratio.' That could have been done using one of the picture mode settings rather than the Menu settings. Certain picture modes override the menu settings.

Ok, I have absolutely NO idea what happened because I'm still importing photos I took on a recent trip - before I encountered the cropping issue with LR. I didn't change any settings (as far as I know), but somehow my photos aren't being automatically cropped upon import anymore. Thanks everybody for trying to solve this issue!

Ok, that is certainly an in camera setting. I also have that option on my OM-D E-M1 and the crop gets automatically applied by LrC at import, it can be changed if necessary.

Clip from the original post “ it automatically crops my images down to a 16:9 ratio even though the original file is not of that size “. It's best to have all relevant information.

LrC is reading and using the in-camera applied EXIF Image Width and Height data as shown below using EXIFTool. This is the way LrC works.

"I switched to 4:3, but it looks horrible on the camera, which is why I'm shooting in 16:9."

Not sure I undertsand what you mean by horrible and why would you use an in-camera 4:3 crop setting when your camera shoots 2:3 crop raw images? We need to discuss this further to better understand what you're trying to do and why.

Since all three laptops exhibit the 16:9 crop issue it's unlikely there's a problem with LrC. Please upload one of the raw files that exhibits the issue to Dropbox for further examination on our systems. Thank you.

it automatically crops my images down to a 16:9 ratio even though the original file is not of that size and it bothers me to manually reset all images before I can start editing them. How do I turn this feature off? Mind you, it doesn't do that on the other devices even though I use the same CC account, settings, etc.!

I assume by other devices you mean other cameras. If so you probably have the 16:9 crop applied in your camera's settings. Other possibility is that you had your camera's mode set to video, which will also apply a 16:9 crop to still images shot with this setting.

As Denis implies, you should double check your import preset(s) or perhaps reconfigure on that one machine and let us know if that fixes the issue. 

Is that crop included in the Default Develop setting for your camera on the particular device?