After going through the research paper I only could think of the theory that they constantly come from the outer edges of the inner galaxy, and are on their way to the centre. But the researchers are not 100% sure about it. They have calculated all these from the past data they had obtained from Chandra Telescope. We need to wait for some time to see the big picture! ;)
I am waiting for @lemouth to have a discussion with him as well on this so that there might be some other reason as well to which our hands haven't reached and who knows we can even know something new about it from him!