The flight appears to have run about 30 minutes late, overall. And it operated with an A321, which is what it was scheduled to have. If there was an equipment swap after everyone had to deplane and reboard, I would think the delay would be much more.
The notes in the flight record (obtained on ExpertFlyer) also don't note any maintenance or specific issues.
"3CLT/ETD0830 CSV-DELAY DURING CUSTOMER BOARDING *0705"
To answer the question in your title "How Common?"...its not very common at all. In the past 2 years of flying ~125K miles, it didn't happen at all to me.
Thanks! Yes the delay was caused 100% by getting off the plane, and then back on. All was normal until then, glad it's uncommon.