Quote from Tony on 04/21/07 at 23:30:17: 
I haven't noticed that yet. You'll have to provide some details. I often add objects to an existing simulation.
My bad, it's when you take an object away.
I made a sim with a bunch of particles, but neglected to delete the original object (sun I guess)
After I had already started the sim (a minute or maybe two), I noticed the sun was still there, so I removed it, and ALL the surrounding particles fled away. It was almost like the sun imploded first, and then exploded. (not visually, but the results would have been similar in a real case)
The movement was definately not from the particles losing their orbit, and I say this because they fled directly outward at a high rate, as if a huge explosion had occured. (not to mention they had not really begun to orbit yet)
I will try to recreate the sim, and then post it.