Nasus Thunderlord's

{quoted}

So how come it is that Nasus' AoE applies Thunderlord's for every tick, but Malzahar's doesn't?

Malzahar's is a standard DoT. Nasus applies a debuff PER tick, so it would trigger Thunderlord's

Because Riot hates Malzahar and doesn't want him to be playable until they rework him.

But actually, probably because it'd be annoying to face getting Thunderlord's off one targeted undodgeable ability. Sure, Nasus' AoE is so large it's undodgeable, but if you're close to the edge, you can at least walk out before you take all 3 ticks.