Subject: Bumping while distilling with vacuum

Hi!
While distilling herbs to get essential oils I use vacuum to avoid too much heat 
that might destroy some of the substances. While doing it, there is a strong 
tendency for the herbs to "bump" up into the condenser. I.e. a kind of 
"explosion" inside the vessel.

My question is: What exactly causes this? The more detailed, the better.
And of course: What can be done to cure it?

Best regards: Ole

Re: Bumping while distilling with vacuum