I came up with it on an online game. I liked it, so I began using it in other places as well.
One aspect of the game is choosing a god faction to support, and I supported one that was associated with nature and mostly represented with the color green, so I wanted my username to have a word that reminded of that, and I thought Leaf did it nicely.
Also, in its battle system, magic was always my favorite combat style, so I wanted 'magic' or something similar in my username. Magics sounded nice, so I went with that.