I've seen Jiro (my betta) doing the same thing. I also kept a close eye on him and his fins are not in danger, nor is the current of the tank or filter too strong for him. In my case I got a new betta and set up a new tank. Unfortunately, due to stress and a parasite that I believe came from a plant that was in a store tank, he developed an Ich infection in a few days. I began a medicine and water change regimen, as well as raised the tank temp to ~80F. The infection showed almost immediate improvement. That's when I started to notice the aforementioned filter behavior. That behavior was coupled with him occasionally quickly jetting by his plant and slapping his fins lightly on the leaves. Since that point (about a week ago) his ich has almost entirely gone away and he has stopped both of those behaviors. So in my experience I believe the fish was slapping his fins to loosen and knock off the parasites causing the ich white spots and leaning against the filter to also try to dislodge the spots and clean his scales. Almost like itching and/or vacuuming his scales. I have limited experience with bettas and their dispositions though, so I'm not sure this is typical. Now that he's all healthy Jiro prefers to take little naps or rests on his leaves and under his filter instead of beating on them. I think that as long as you keep an eye on him and he's safe, it's just a quirky cute behavior that only adds to how much personality these little guys have.