I was just goofing around and playing support Kalista and I notice I was getting the gold from minion kills that I didnt last hit. I played a second game as Kalista support after. I believe it happens when I recently applied rend stacks to minions, and then someone else comes along and kills them. I'm not 100% sure if it works on the melee minions or not. I only noticed it when it happened on the caster minions.
That's a feature of her W passive. If she or her oathsworn attack a minion, that minion is marked; if the other one of the pair attacks (i.e. if Kalista marks the target, then the oathsworn attacks it, and vice versa) it takes extra damage. And the game is coded in a way that if that extra damage kills the target, Kalista takes credit for the kill.
EDIT: Quoting from the wiki
SOUL-MARKED - PASSIVE: If Kalista and her Oathsworn ally attack the same target within 2 seconds of each other, the target will take additional magic damage equal to a percentage of their maximum health. This effect cannot occur on the same target for 6 seconds. Damage is capped against minions and monsters. Soul-Marked will execute minions with 125 health or less. [...] Additional Information: {...] The damage dealt by Soul-Marked is credited to Kalista and will benefit from Kalista's magic penetration and spell effects. In the event that the Oathsworn scores a kill using Soul-Marked's damage, they will get a message stating "Kill Secured" in place of the usual gold pop-up.
EDIT2: Basically, because they intended to make Kalista a very teamwork-reliant ADC, specifically communication with her Oathsworn ally, they gave her an ability that encourages her and her oathsworn to communicate and attack the same target (The duration of the 'mark' is very short), and because they intend for Kalista to be an ADC, they made it so she would get the gold for it.
