Is that what you actually flew on? If so, it was doubtless what is called "subservice", where a carrier which is unable to service a flight charters an aircraft from another company to make the flight. You would have had ATA pilots and crew. The flight would likely have operated with USA3000s call sign and flight number.
The likely reason would be maintenance on the aircraft that was to be used by USA3000. Given that USA3000 had enough time to call in subservice, it must have been a period of at least several hours since the USA3000 aircraft went out of service. Most likely, the ATA aircraft would have flown all the flights that day that were to be made by the USA3000 aircraft -- perhaps for several days if that's how long it took to get the thing back in service. Some other possible reasons for using subservice: (1) aircraft is delayed coming back from heavy maintenance and no spares to cover; (2) crew shortages cause available crews to exceed monthly flight limitations or required rest periods and subservice is brought in on one or more lines to get people "legal" to fly; (3) schedule expansion results in increased need for aircraft and supplemental aircraft do not arrive on time or make it onto certificate on time; (4) irregular operations due to weather or whatever have the whole schedule moving well behind and additional lift is needed to move out "trapped" passengers and get the system back on schedule.
I haven't heard of massive use of subservice by USA3000 recently, so this may have been a localized issue. ATA is a very flexible charter operator, particularly as to things like catering: did you get whatever meals and such that you would have had on USA3000? Did you fly back on USA3000 metal?
JetBlue used North American not too long ago for a flight from JFK to somewhere in Florida, getting out a bunch of extra passengers that were there due to irregular operations. They provided all the usual JetBlue catering and services, and put some JetBlue service coordinators on board as well. I saw some posts on here that suggested that without their eyeballs glued to the TV sets (which NAA of course didn't have), folks had time to look around the plane and some bitched that it looked "old" or "dirty". Shows how much people don't notice when they are focused on what my friend Al Vitale calls "the electronic babysitters". Next time they'll have to charter from Frontier.