My betta flares all the time when I'm in the room. Either he's flaring at me or at the package of food that's by his tank...I'm not sure if it's normal but at least mine's not the only one.
Do you have a background on your tank?
HOw's the lighting in your house? It could be that if the lighting outside his tank is darker than inside (or really any type of reflections, etc.) could be turning his tank into a box of mirrors. Try adjusting some lights - or take the background off.
2 of my bettas literally go crazy and jump out of the top of the water halfway if I have backgrounds on their tanks.
I bet it was!
When my boys are irritated, then get all puffed up, then the raise their head up to the surface of the water and do sort of a side to side dance. Their ventrals point almost straight forward and then they jump up out of the water and flop over on their sides. It's kinda funny to watch, but when I catch them doing it I adjust the lights.