For those who haven't seen the ship before, ReiAli was popular back in the series' early days, but dropped off at some point after 2010, probably because Alice hasn't shown up in a game since Hisoutensoku (but she still appears in the official comics). Some people still write/draw it every now and then, but most stories you'll find here and elsewhere date back a ways, so greener fans are unlikely to see it.
Edit: It has its roots in her pre-boss dialogue in Perfect Cherry Blossom on Reimu's route, where she greets her with "long-time no-see," while Reimu counters with "we only just met." If you consider Mystic Square (Alice's first appearance) canon, then this can be construed as Reimu having forgotten their fights in that game. The fandom being what it is, this was all that was needed to launch the ship.