First of all, I would associate this with increased anxiety. What is called "nowhere to put your hands." Doing this can help reduce anxiety. By the way, regularly looking into a smartphone may be from the same opera.
It can also be a demonstrative gesture of dissatisfaction, distrust or pressure. Imagine a boss who does this when you try to explain why you were delayed by five minutes.
And the last thing is provocation to aggression. Here you can not only tap your fingers, but also click, hum a song, etc., when it is inappropriate.
Why annoying - rhythmic sounds attract our attention, can interfere with concentration and therefore cause irritation.
But in the end it all depends on the context. Apart from him, it is impossible to say exactly what this behavior is connected with
This is not love or pleasure. This is a manifestation of intolerance and irritability. Thus, we signal that we are bored, we are nervous, angry, in a hurry. This signal transmits negative to others. And nobody likes the negative, so they get annoyed.