What was this?

I was watching an episode of Space Voyages on the SMithsonian Channel last night
and early in the show, they showed the launch of a modern era rocket. Could have
been an Atlas or Delta, couldn't really tell. But as soon as it started to take
off, there was very visible rocket exhaust coming from the top, at the payload
area. I never saw anything like that before. It wasn't an emergency escape
system, as it didn't pull the top off the rocket. Any idea what this could have
been?