Most of the time you will have a 1 in 3 chance of losing XP if some one downvotes a node that you have written. If the reputation of that node is between 3 and 4 times the value of $NORM, then the odds are only 1 in 4 that you will lose a point of experience. Once the reputation of the node is over 4 times the value of $NORM, then there is no chance that you will lose a point of experience.

When some one downvotes a node, it could be for a variety of reasons, usually it indicates that someone happened to disagree with your point of view. For the most part, I try to avoid doing that, instead reserving a down vote for a trollish or incorrectly formatted node.