If a question contains context that is factually correct, but not absolutely necessary for the question. Is this a valid reason to close a question?
Please do not answer different questions and limit yourself to the question posed.
Imagine this question:
Although it has not been determined why Sullenberger was able to land the plane in the Hudson, the general assumption by the media is that it was because of his experience and training.
Why don't aircraft have floatation devices in their bellies?
The first paragraph is true, but doesn't at all change or constrain the answers that you'd get. So, edit away the first paragraph.
That doesn't make it off-topic though. What would make it off-topic is if five people believe it is off-topic and vote as such.
It's a little tautological, but basically anything that enough people vote to close for is a "valid" reason to close.
Unnecessary context might be a reason to close the question, depending on what that context is. If that context is there to promote a service, a product or some standpoint, I can see the question being closed for that reason.
More likely the context will be edited out if there is a valid question.