Seems to be a new marketing strategy. This had never happened to me before but a while back I also got an email out of the blue from an unknown source wishing me a happy birthday. It turned out to be from a car rental agency that I had rented a car from last year. They record the information from your drivers license for the rental contract so they have your date-of-birth and they obviously keep it for marketing purposes.
How anyone working in a company's marketing department who has half a brain can possibly think that a random emotionless computer generated birthday greeting from a faceless company will be received by me with anything but disdain as it is nothing more than irritating junk mail that has to be dealt with can possibly be a good marketing strategy baffles me.
And on that note, from someone whom you have never met or spoken with and who is only a computer acquaintance- allow me to wish you belated happy birthday!