Maybe from either extreme, this doesn't seem to make any sense, but from the point of view of one who has recently *become* atheist, or who is still close enough to religion to still resent the years it was foisted on them before they were old enough to consent, this makes perfect sense. I drifted from Christianity very slowly, and broke down in tears when I realized it was gone from my life. I mourned the ideas I couldn't force myself to believe anymore, the way I mourned dead relatives and friends. I can very much see how somebody would want a formalized split to mark the occasion--either in joy or sorrow. Plus, Oscar Wilde was right about a few things. One is that, for some people, the artistry of real life can help the person process it. Performing some ritual, by choice, that mirrors the one that was forced on them could provide some people with a sense of symmetry and balance that helps ease their passage into what may feel like a new life, or may help them feel like they have fully cut the ties to an organization.
For whatever reason a person might chose to do it, it isn't a decision that needs to 'make sense' from either a theistic or atheistic point of view. It's the sort of decision that a person would make from from feelings sentiment, personal symbolic significance and aesthetics. Like an initiation, or eating your late mother's favorite meal on her birthday, or something like that. There's no reason why anybody 'should' do it. But some people may find comfort in it all the same.
