A single dark corner like this is usually caused by something physically blocking part of the frame, or less commonly by an intermittent shutter issue.

From the answers, the first thing to rule out is obstruction: an incorrectly fitted lens hood, a hand, hat brim, or something protruding near the front of the lens or inside the camera. Even though no hood was attached, this kind of shadow most often comes from something intruding into the image area.

If nothing was blocking the lens, an intermittent shutter problem is plausible, especially since you couldn’t reproduce it later. Cold conditions could make a mechanical part move more slowly, which might explain a partial exposure loss. Checking whether the affected images were shot with the mechanical shutter would be useful.

Because the issue was intermittent, it may be hard to confirm at home. If it happens again, note the shutter mode and inspect the camera/lens mount area for anything out of place. If there’s no obvious obstruction, the safest next step is to have an authorized service center diagnose the shutter.