Or you're trying to upload a really long video?
Please keep in mind the bold and all-caps message on the WEBM upload page:
PLEASE DO NOT UPLOAD VIDEOS OVER 1 MINUTE LONG IF POSSIBLE!!!
Also if you are trying to upload a MP4, which tend to be larger in size than WEBMs, I would recommend converting to WEBM first because the way the site converts MP4s to WEBMs just makes them look blocky and terrible...and the WEBM will be smaller and easier for you to upload.