After thinking about it for a little while, I've come up with something
better. I wish I could draw a picture but I will try to explain. As the
shock wave from the bolt is first generated, each portion of the channel
produces an outward traveling wave at the same instant in time. But the
parts of the wave originating very close to the observer have a uniform
amplitude. This means that the wave initially travels perfectly
perpendicular to the bolt, expanding in a cylindrical fashion. As the shock
wave breaks up and decreases in amplitude this cylinder begins to break up
as different parts of the wave go out of faze with each other due to the
unevenness of the channel.

When you are standing very near the bolt, the shock wave will travel
perpendicularly to the bolt so that you only hear the closest part, while
the sound from higher up passes unheard over your head. This is the laud
snap. A split second later you will hear the more rumbly thunder
originating from higher up. This sound has already broken up and gone out
of phase, so portions of it can reach your location since it no longer
travels perpendicularly but now spreads in every direction.

I am pretty sure this is the answer to your question. However, I would
appreciate some feedback or corrections if someone knowledgeable stumbles
across this message.