Hmm. The impulse to go and yell at the HOA strikes me as a bit drama queen, to be honest. Yes, it's disappointing to not be picked for the team, especially if you thought you would be the most obvious candidate. However, since the aunt wasn't privy to the decision-making process, it seems unlikely that she would know that it was a case of the association dissing her.
In my HOA, people generally decide whether they want to be a candidate and will say a few words at the yearly meeting. Then, the members vote. If that's the case for this woman's committee, then winning the popular vote could happen for any number of arbitrary reasons (the other guy's voice was annoying, people preferred the way she was dressed, the winner had a more confident/competent "vibe". . . ). In nominating and electing a replacement, the committee probably looked at individual qualifications, previous involvement in committees, etc.
I guess that I don't understand why this lady feels like she was snubbed. I understand that in previous cases, the committee sometimes selected the next person on the list, but I really don't see any particular evidence that the deviation was a deliberate insult. It sounds like the election of replacement officers is closed (that is, committee members nominate and select someone rather than presenting it to the entire HOA), so I don't see how the aunt should have "had a chance." (Who knows? Someone may have nominated her and the others decided another person would be better.) The main miss, I think, was not informing her one way or the other. Everyone on the HOA should have been told that an officer had quit and that a new one would be elected. When the replacement was selected, the committee should have made a general announcement.
To answer your general questions at the end, I think that being gracious is always best, but that it is fine to request clarification about why you weren't selected. Unless you have good reasons to think that someone was being discriminatory or otherwise unfair, it's best to frame it in a "What should I do to be more attractive as a candidate if a similar opportunity presents itself in the future?" kind of way.