Spike in bandwidth at beginning of stream

[trek1413]

I have been graphing the bandwidth of video streams. All of the videos spike in bandwidth for the first second before it returns back to a normal rate.

I have an example at:
http://www.pages.drexel.edu/~rtm92/sample.pdf

This particular graph shows a udp/rtp stream of a 20 second long mpeg4 that i had on loop.


Any thoughts of why this might happen?