Hmm.. yeah, it's hard to say why if you don't know the game. In Ragnarok if I go AFK it's either cause I'm waiting for someone to show up and want them to be able to find my afk avatar, because I'm buying/selling goods and don't want to hover in the meantime, or I might be waiting out some variety of timer in rare circumstances. Also, sometimes on priest I just hang out and randomly buff passer-bys who ask, but otherwise might look afk.