(just to be pendantic, but obviously what you're asking is "why does it take so long to erupt relative to the time it takes to jet in the air?" because of course, it takes a split second of real time for both these events to occur.)

if you look closely, it looks like the rip causing the final eruption happens in a different place than the "bottom" on which the kernel is resting when it's on the "ground." It kind of rotates in mid-air like the space shuttle does (or did) on its way up. hence, I don't think the two are related.

that said, I'm not sure, though, what would cause the kernel to fly into the air. I see no steam escaping.