Help with ffmpeg - stretched videos

[svmariscal]

Hi, I am scraping my arcade roms with Skraper and downloading content from screenscraper.fr. Some of the downloaded videos (I checked normalized videos in Skraper) won't show in retropie, so I've been running ffmpeg to change them to yuv420p format.

Here's my command line:
ffmpeg -i <original_file.mp4> -vf format=yuv420p <destination_file.mp4>

the problem is most of the videos show up fine in retropie after running them through ffmpeg, but a few ones end up stretched vertically (or squeezed horizontally depending how you want to view it), like they have half the horizontal size they shoud have.

For instance, frogger shows up like this:

This only happens to a handful of videos (all of them older roms like frogger, galaxian or phoenix), like I said most of them preserve their aspect ratio just fine after re-encoding them.

Does anyone know how could I force aspect ratio with ffmpeg so this doesn't happen?

Thanks.