Here are some details on the video, from the description on YouTube:
This is a timelapse of the 2011 "March for Life" in Washington D.C. This video is 599 still images taken every ten seconds for a total duration of 99 minutes. The official start of the march is designated by the banner you see at about 13 seconds in. Everyone in front of that banner jumped the gun. That is why there is a sizable gap just before that banner.
You may also want to check out a similar time lapse video, also from last year's march. I will watch for a time lapse video of this year's March for Life, and I will post it here if I find a good one.
(Thanks to Steven Greydanus for the link to this video, which he provided in one of the comments on his March for Life Live-Blogging post at the National Catholic Register.)