It may have been a glitch, but there are certain strange responses to NS lookups which may or may not indicate an underlying problem. You can do a full report on any domain at dnsstuff.com, and every one of your domains has at least one problem reported. These may not be serious enough to stop dns from working, and may be something to do with the way the unusual way the hosting company have set up their dns. They may have a legitimate reason for things looking a little unusual, but it's worth closer investigation.
I have run the domain reports against our business domain and a few others, and they come out 100% clean, so the dns reports are not one of these things where no-one ever gets it perfect. If you notice some of the explanations in the domain reports, you see things saying that dns will function, but there could be delays. Too many errors like this, and the delays could well turn into complete failures.
There's usually no reason why dns servers cannot be set up to "play by the rules" as specified in the rfc standards, so if it were me, I'd be asking my hosting company to explain the unusual reports.
