erm.........HIV is quite stable...........just that the cure is still elusive...........and the virus hit hard on the immune system causing immunodeficiency
influenza(H1N1...........H10N7....) can be unstable........especially influenza type A due to antigenic shift and antigenic drift.........

and why that happens...........because the components of the virus(including antigenic structure) fuse together in the host cell randomly......there's also a possibility of integrating parts of the host cell as one of the structure of the virus

as for why now..........it happens all the time actually.......human's traveling around the earth also gives the virus a hand in spreading the infection hence the higher chance of exposure to the virus and the media which reports more often of the occurrences