The problem isn't so much that they're afk (necessarily), but that they're likely a bot that's either programmed to run around the map on their mount- or intentionally trying to lose quicker and evade the afk auto-kick by being a drain on their team or giving the enemy teams a source of Battle High.

It can be difficult to notice sometimes, but if you do see someone with unusual behavior like repeatedly running to the enemy territory alone and dying at a abnormally quick rate between respawns, chances are this is what they're doing. It's not always a bot, but more akin to someone intentionally letting themselves be killed as if they've given up and want the match to end sooner. I've called-out a few people for this behavior before, and they usually make a lame excuse and start acting like they're contributing. Unfortunately, vote kicking isn't always a solution, because the majority of your party may be apathetic, and at that point- what can you do?