The HF Reject adds a 50kHz low pass filter to the trigger path which is going to add quite a slope to a fast edge. On a MSOX3104A, I can get it to trigger up to 20us after the actual transition, depending on the trigger level. And there is a ton of jitter.
Wrong trigger option. Simply don't use it for fast signals.
If you are using it to counteract a lot of noise, consider using the Noise Reject option which increases the hysteresis, or improve the probing to get a better edge on the external input.
Or cook up some other trigger configurations for burst or pattern to trigger on the two displayed signals. They look pretty clean, relatively speaking.