|Ants farming aphids|
|Ant tending aphids|
Now back to the question, "what is attracting these ants?" the answer is, "aphid honeydew".
The ants weren't milking the aphids fast enough and so they just "let 'er fly". The honeydew droplets landed on leaves lower down on the bush where other ants were imbibing the sweet honeydew drops.
My backyard is a bonanza of astounding natural phenomenon.
Here is a link to an article on how ants herd aphids.