Exhibit A: Lux hits an opponent with a spell. Her passive is marked on them and then she autos them, setting off her passive and then thunderlords.
In the sequence the opponent has been damaged by Q/E, an auto, (P), and then finally thunderlords.
Exhibit B: Varus autos an opponents. His W passive is marked on them and then he uses a spell, setting off his W passive. Thunderlords does not happen
In this sequence, the opponent has been damaged by an auto, a Q/E, W passive, but not thunderlords.