Not saying it had anything to do with X-files, but also when you shot something for TV but were not entirely sure if the capture would ever go to the movies, you protected the wide frame.
Also, if you shot a movie but wanted it to look good in TV later, you put the most important action somewhere in a 4:3.
Not quite but close. Titanic, like a lot of movies of the 90s and 2000s, was shot in Super 35. In Super 35, the image is ~4:3, but it requires optical printing or scanning to produce a release print, since the image is both not the correct aspect ratio and also occupies the area used for optical sound.
So it was not "cropped in the theater"—the theater got a standard anamorphic print. To go from the Super 35 negative to the anamorphic print, they both cropped and optically squeezed the image (in the case of the non-vfx shots), and scanned, cropped, squeezed digitally and printed back to film (in the case of the vfx shots). This was a few years before they did full "digital intermediates."
35mm film is more square than rectangle. When you shoot wider aspect ratios, the whole frame is exposed. The eyepiece on the camera has lines on them to allow the DP to see the framing for the desired aspect ratio vs the whole frame. So it wasn't just a Titanic as example, it was pretty much all film was shot like that.
When digital cameras like the ones from Red came out, you can tell it the aspect ratio so it only saves the active pixels of the full sensor and ignore all of the out of aspect pixels. That's a brave operator doing that, and I've only seen it in the wild once.
Also, if you shot a movie but wanted it to look good in TV later, you put the most important action somewhere in a 4:3.