The bar goes red when your BitTorrent can't connect to the tracker OR when the tracker sends back an error.
You get a hint on the cause by looking in the lower BitTorrent window part with the torrent's details under the "Trackers" tab in the line with the tracker announce URL for the message in the column "Status".
That message can be long, so you might need to expand the width of that column to see the full message.
Same examples:
time out -> connection not possible
bad ratio … -> to many downloading torrents at the same time (see downloading rules)
unregistered torrent -> the torrent has been removed (there are many possible reasons for that)
Your BitTorrent still has the IPs of peers in memory (until you stop it) and therefore can still download from these it knows.