Long story ... Once Upon A Time a mate of mine used to call pieces of software after his girlfriends, so to take the piss I called something Gertrude. Plus I rather like wombats (although they're not the brightest of animals).
So, when I came to sign up to PPRuNe nobody but nobody was using their real name, and to do so would have looked decidedly odd. In protest against this weird convention I made up the daftest name I could think of in my then current state of sobriety, and there we are.
Not that posting under other than one's real name actually hides who you are. I once asked in another place, nothing to do with aeroplanes, where I post under my real name, "so what's my PPRuNe handle then?" and someone (not a PPRuNe regluar) had worked out the correct answer in less than an hour.