After all, you can more or less divvy up the possibilities as:
A causes B B causes A both A and B are caused by C
There are at least two more possibilities: A and B are unrelated, but happen to be in sync for a while, and the data was collected wrong in some way.
You could say that "A and B happen to be in sync for a while" is possibility 3, where C is the passage of time. (Unless by "happen to be in sync for a while" you mean that they appear to be correlated because of a fluke.)
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