There are lots of groups out there that hate the USA to irrational levels. Just what on earth could the US gain from sinking the Kursk? Often that is what you need to consider.
What would those people gain from this action?
What are the risks if they are caught?
Do they have a history of doing such a thing?
Sure I too can type up a professional looking report that a UFO was found hovering over the Kursk before it was destroyed as well.