Probably didn't think he got a sendoff that was big enough, with enough fawning and non-stop attention to fit his ego. Once the "Will he or won't he" and "Oh, please don't go Tom" hype and frenzy went away, he probably decided to make today's announcement to get back in the limelight he craves.
Toward the end of last season Ben Rothlisberger's (sp?) retirement stole a bit of his thunder, and he probably would've liked to have seen a Superbowl win be his "swan song" and perhaps he thinks one more season may give him that chance? Ya never know. Wasn't it Bret Farve who retired to great fanfare and then un-retired years ago?