At one time it was called free floating anxiety. I'm sure it has a cause but it usually isn't obvious. And when you can find a cause your mind will frequently assign a cause whether it's warranted or not
I don't have any foolproof methods. I can recall my mind assigning some really ridiculous causes for the no-specific anxieties. I would examine those "causes" and try to analyze the reality. And sometimes I'd just muddle through and try to let the anxiety roll off. It isn't easy I know