"Shia fighters are said to have used off-the-shelf software programs such as SkyGrabber to capture the footage."[1]

Why the BBC calls this a hack [cause it sounds cool I guess] I have to idea. This is a classic interception case. Get a program (SkyGrabber in this case) and start receiving the broadcasted satellite communications. The US should not have had insecure satellite communications in the first place.

Links:
[1] Iraq insurgents 'hack into video feeds from US drones' - http://news.bbc.co.uk/2/hi/middle_east/8419147.stm