The phrase "unfathomable reason" means that the reason or cause behind something is very difficult or impossible to understand or comprehend. Full definition
It's futile to look any deeper, for example, to consider how policies such as meddling with interest rates might have influenced investment decisions, because, even though the real - world economy involves millions of individuals making decisions for a myriad of reasons, the individual actors within the economy supposedly form an amorphous mass that shifts about for unfathomable reasons. (tsi-blog.com)
It is true that those marks are all made by his own hand, not because he has an objection to studio assistance in principle, as he explains, but for more unfathomable reasons. (apollo-magazine.com)
Unfortunately, on various occasions, Apple has decided, for unfathomable reasons, to break with convention and create implementations that are more tedious than they really need to be. (guidohenkel.com)