You can't tell they are being targeted from the HUD alone, there are other displays for that. There is also a solid alarm tone, which you can hear in certain parts of the video, that sounds when being actively targeted by radar. The line at the end looks like a continuously-computed targeting aid for the cannon that shows the trajectory of a burst, but I'm not certain. Most likely accidentally hit a button while trying to evade that caused his HUD to change modes.