As far as I know, it only happens when you heal in front of them. Did you heal near the Smilodon? If you are level 90 and heal in front of a Goblin near Bastok in the present, they'll attack you for example.
The only "Too Weak" monster that attacks no matter what are the Detectors in Ve'Lugannon Palace and Defenders in the Shrine of Ru'Avitau. They are aggressive to sound. I can't think of any others off the top of my head.