I've had this problem in the past with the Opera browser. and maybe with other programs too. It's rare. You just kill the funny process, and start it. I'm sure it has happened to many people and doesn't bother them. It's like asking why a program crashes.. it crashed.. dealing with it is what one can do about it.
One of the instances crashed. So it comes under the category of just a crash. But this has the added thing that a new instance won't start unless the old one is killed.
Sometimes i've thought it RAM related.. 'cos I think I may have had situations where a program won't start until I end task a different one. But this is probably not the case here, especially since wmplayer is probably small relative to the available ram you probably have.
I'd be interested to know the cause of that. Maybe a windows programmer might know what is going on. So could be made into more of a stackoverflow question.
Usually with crashes though, you don't know unless you are can see the code and what's happening in it. Not something anybody other than microsoft can do with windows media player. But since the not being able to start another instance is a problem i've seen in other programs , maybe this is a type of crash that programmers may know a little about. though I doubt it'll be much use to you. You've figured out how to deal with the crash.