There are a hundred reasons as to why a flight might be delayed or canceled, from mechanical issues to congestion issues at the airport. We suspect that this is quite a unique one, though. Do airports even have an official code for 'there's a polar bear on the runway?' If they didn't before, we bet they do now.
To be fair, this isn't likely to occur at most airports. This photo was taken at the Wiley Post - Will Rogers Memorial Airport at Alaska, where polar bears are a reasonably common sight.