People are used to 'normal' openings with a very quick battle over the central squares. Stuff like this delays that and leaves people guessing. They might set up for you playing e5 eventually and when you go for d5 instead they'll have misplaced pieces.
It's psychological as well, like a boxer dropping his hands low and taunting you. some people get angry and want to mate you immediatly, thus overextending themselves.
Not that I recommend playing 1..a5, 2...b6, 3...Ba6 objectively though, it's creating weak light squares and then exchaning the bishop defending those.
Lately i get white pieces i play dumb and lose, so i challenge for a rematch, and to punish myself I play 1... a5 I didnt even know what it was called i had to look it up. It's called the Ware defense, and the same guys i just lost to I pwn with this. I havent got any lines memorized, i dont know any theory, i havent studied any games, I dont know if it is psychological or what but it almost makes the win worth the loss when my opening move says "I can play any garbage and beat you, you got lucky last game". I am 4 for 4 this month with this ridiculous defense against players rated 1407-1523
Why do you think it is working?