If it is not an alpha act then generally something is going on mentally or physically.
Dogs don't do things out of spite or malice, they do things out of a physical illness, or instinct.
The fact that it is only your son's bed can mean that either an odor attracted her to the area or this is a place she felt comfortable in going when she had a need to go.
Once something like this happens, then they are drawn back to the same spot the next time.
Unfortunately you may not ever know the reason she is attracted to this area unless you do a little detective work and to do this then you would have to keep the door open and watch her and her previous actions 100% of the time to see what lead up to the problem.
Take notice of what is going on in the home at the time and who was in your son's bedroom prior and what was going on in the bedroom. Take notice of what the bedroom looks like.
Is your son a neat person or does he leave clothing around? Is his bedding being washed once a week or does he let it go longer?
Little things are important to finding out a root cause.