I am not sure. Somehow the confirm page must have thought the order was denied even though it was not.
Are you able to reproduce the problem? Where there any errors on the page during the failed transaction?
It could be something as simple as your web site going down or Authrize.net taking too long to respond causing an error. It could have been your database was temporarily offline. It is really hard to say. That is not expected behavior in any circumstance.
The most important step in fixing a problem is being able to reproduce it. There must have been something different about that third order that caused the problem. Once you can reproduce the problem you can work on it and test to see if it is fixed. If you can't reproduce a problem there is no way to tell if it is fixed. Luckily software problems are almost always reproduceable once you figure out the source.