Or the gate agent simply screwed up and panicked. I've seen that happen. "Oh lawd! I'm oversold by 6!" She got like 10 volunteers and took 'em all. What they're SUPPOSED to do is wait and see if there are any no-shows or misconnects, then they only take the first X amount that volunteered. For example, let's say you have 4 volunteers on a CRJ, that's booked to 55. Because of 1 no-show, you're now down to 54. Now you have a couple of late flights triggering 4 misconnects. Bingo. You're down to 50 and don't have to take any volunteers. You thank the people, but then you inform them that you won't need to remove them from the flight. They go on their way, but they don't get the travel voucher or the night at the hotel since you didn't have to deny them boarding. What normally happens is gate agents freak out and don't want to get screamed at or delay the flight. So, they generally take more volunteers than they need IF they get them.
What the USA Today article failed to mention is that ALPA mentioned the staffing problem to NWA management months ago.....