It's more than just the hand gesture, of course; he's doing a full-body jump-scare reaction, but why the specific, "I love you"/Spider-Man-web-shooting hand motion? He does it at least twice in the third volume:
So it's not just a quirk that Takahashi threw in one time. I'm strangely fascinated by this; is it some sort of ritualistic warding off of evil spirits? Some kind of recognizable gesture of the type that are so popular in Japan? An in-joke of Takahashi's that I'm not privy to? Or just something goofy that doesn't deserve this much thought? If anybody can illuminate me, I'd be quite happy. Or I could just go on imagining reasons until my next obsession comes along.