Did those mobs chased after you
before or
after you were able see their red health bars (assuming you have show-enemy-healthbars enabled)?
I ask, because
originally: if the level difference between your character (any playable character) and a aggroable-mob is high enough, the mob will aggro whenever your character is
within 40 yards of that mob (though no higher than that, no matter how large the level difference is). That's also precisely when you're able to see the mob's health bar over that mob's avatar (and you can see those even as a ghost [if the mob is close enough to your corpse]), though you'll be able to see their avatar (or /target them) up to 100 yrds away (yrds as in in-game yards, like for range of spells, etc.).
TLDR on Originally (or, did these change with 7.3.5?):
-Max aggro range: 40 yards.
-Max range to see their health bars over their avatar: 40 yards.
-Max /target-able range: 100 yards (out in the world. Different story in instanced-pvp).
The
RangeDisplay Addon will also help to check if the max aggro-range/etc. was changed or not.