Applying conditions to ambient creatures puts you in combat, but just using direct damage doesn’t. Consider: if I kill a frog with my warrior’s mace auto attack, it never puts me in combat. If I kill a frog with my warrior’s greatsword auto attack or sword auto attack (notice bleeding and vuln come with those), I get put in combat. This is also reproducible with other classes/weapons.
Killing ambient creatures is a great stress reliever and good way to get weapon master achievements while leveling. I thus humbly ask that this inconsistency of triggering combat speed against ambient creatures be changed so that applying conditions to ambient creatures does not put you in combat.