It is evolutionary. People who are tense and anxious probably are more aggressively searching the landscape for perceived threats. Therefore, they tend to see faces (and threats) in harmless scenarios.
This has some marketing implications, right? Are there package designs that could be perceived as faces, which in turn could cause some consumers to unconsciously reject those products out of fear? Could there be cues in advertising that unconsciously freak people out because a random inanimate object resembles a face?
If you are into this sort of thing, follow @omghiiii on Twitter. Some of the images posted there are just creepy.