Are you a hunter?
Colossus Slayer is applying as a separate damage instance in the combat log. It also shows as a separate floating damage number on your targets avatar for this reason. Not sure why it was implemented this way, but its really buggy at this point so I imagine it will get reworked.
Currently, as it stands, it triggers completely randomly. Sometimes with Hunter's Mark, sometimes not. Sometimes at full hp, sometimes not.
