No red flags that I know of! No cheating, criminal convictions, major gaps in standard extracurriculars (that I know of). The only thing I can think of is they did not think I was a good fit. I am research oriented and interested in academic medicine, and my essays and activities have reflected that. I know UCD has good research opportunities but I don't think that is their emphasis. They seem to like to talk about their family medicine, rural medicine, and diversity. I grew up poor and lived in a 90% Hispanic rural community and my parents are both HS-drop outs, and I have a journalism/humanities background... but perhaps that doesn't fit their definition of diversity (I am not URM). Perhaps they felt I was not a good fit for their mission.
Thanks for the kind words. It has shaken my confidence a little but sometimes we need to be humbled. I certainly don't want to be in the mindset that I am entitled to interviews. 🙂
Edit: If you're applying, I wouldn't worry about my stats as an indicator that UCD is hyper-selective about stats. My MCAT and GPA are in their top 90th according to MSAR, and the combination of my stats may be even higher. It's wiser to just look at how your stats stack up to their posted averages, which are much more reasonable (32 or 33 average?). Good luck!