As has already been discussed, this did not happen in Vanilla. Training mobs onto someone else required the mobs or the other player using an AoE. That is very different from what is happening in Classic.
If you are fighting a mob and other mobs run by chasing another player, them passing close to that mob can add you to their aggro table. It is similar to when humanoids run away and pull other packs to you by running near them.
Also as mentioned them screeching or doing any aoe while running by can agrro you and sometimes it’s not very apparent. Think Kazzak in Stormwind shadow bolting everyone in the city without them ever touching him.
If you have a passive buff that regenerates health for example, coming in range of their (the mob) aggro’s range even if you did not engage in combat while they are focused on another player can cause them to attack as you get put on their ‘list’.
Come to think of it, that may be Everquest I am thinking of.
If mobs are in range on another player and you cast a heal on yourself. You get out on their aggro table and are subject to attack.
Not sure if this is accurate or not.
In EQ, breathing gives aggro