It means either of the following things:
[*]We're receiving a lot of traffic, to make sure the site doesn't go offline we stop serving pages untill server load goes down.
[*]We're receiving a (D)DoS attack, ---^
[*]Some process on our server is taking a lot of processing power, this may be Backups | File Scans (With a lot of uploads, this can take up a large portion of the cpu) | HTTP Flood (see above) or Frequent file uploads. (Creating topics, etc.)
So yeah, sometimes this will happen.