No one really knows why they tag an individual for further processing. Most of the times it’s a system prompt although the VO has discretion even if the system does not prompt. That being said there is no issue travelling to the US when DQd. At the interview they will ask you about your recent visit to the US, what was the longest time you stayed, purpose of the visit. Also CBP can ask you on purpose of visit. In my case I travelled to US for 2 weeks just to get my TN renewed from within US rather than at the border. There were no issues with CBP/USCIS and interview. They also ask you at the interview if you have ever been denied entry or visa refused. I was refused entry once on my initial TN entry for a missing document and I informed the VO about the same. My Initial TN was eventually approved when I went back with that document. I did provide specifics about date and land border of that incident.