Mine do that too. The last boy I got flares at everything, as soon as he sees me he flares and swims all over the place. Then if I come too close he freezes and sits in one of his plants motionless. He's a goof!
My betta Scotty flares at the wall, at me, at the TV, his plant, the air, etc. He just swims and flares, swims and flares. The one and ONLY thing he does not flare at is the camera...Drives me crazy! Here I am wanting to take a picture of how fierce he thinks he is and he gets all smooth and pretty for me! Crazy fish!
Sometimes they can see their reflections in their tank and flare at that... or sometimes they're like my Abrid and just flare at anything no matter what