But then you have to face the facts - H1N1 found itself a phenotype that it liked. H1N1 properties worked for it. H1N1 settled down for a comfortable long wait. In other words, in 1918, one version of the flu settled down to a more or less stable form which was very successful for it, allowing it to wreak havoc in the early 1900's. There was no need for it to mutate to a further form. And then it committed the cardinal mistake that pathogens can commit, it started killing off susceptible hosts faster than they could regrow. It burned itself out, all except for some samples thoughtfully kept by scientific and medical staff at that time.
And now, we suddenly have a recurrence of a stable and (in virus lifespans) many generations extinct virus. Since it's a stable form, we know it didn't just "spontaneously evolve again" from existing viruses. That's like having specially bred domestic chickens, something comes along and kills all domestic chickens - and then suddenly, spontaneously, the wild chickens in India and Asia start producing domestic fowl mutations. Evolution doesn't retrace steps.
There's evidence to indicate that once an evolutionary step is made, the organism burns the bridges behind it, making it much harder to regress to an earlier form. And while there are some quirks of evolution, we're begininng to see more and more that organism don't just retrogress. So you can forget saying that H1N1 just threw back from newer strains of the flu virus.
Furthermore - whether deliberately or unintentionally, there have been several releases of biohazards from laboratories around the world. Most immediate to me is the 1995 release of rabbit calici virus (RCV) from an island facility (Wardang Island) off the South Australian shore. There was some discussion whether or not this was actually a deliberate and clandestine release to help farmers hard hit by drought and rabbits denuding their fields, or a genuine accident. It doesn't matter either way - it perfectly demonstrates that inadvertent and/or deliberate releases are not only possible, but can and do happen.
So we're in the unfortunate position of deciding, not between whether it was an act of nature or an act of man, but instead that we have to decide if this act of man was an accident, or a cold calculating and deliberate release. Welcome to Human nature...