Dremel question

There is another dremel question floating around, but I don't believe it addresses my question. I use my Dremel to sand tight corners in wood projects. But the behavior of the Dremel concerns me. When I use it, put it on 'high' and sand away the stairsteps that the jigsaw leaves in tight corners, I notice that after removing the Dremel from the surface of the wood that it begins to speed up alarmingly - running extremely fast and accelerating. In a situation like this I'll switch it to low speed and wait for it to drop down. But even on 'low' it will rev up after removing it from the work piece. Is this normal behavior, is the Dremel speeding up to cool itself down (I notice vanes in the body of it attached to the motor), or is this some condition that indicates a problem with the Dremel. I don't believe myself to be pressing down on the dremel attachment too much - after all I'm sanding wood not grinding rusty steel.