This has been a mystery and I've not run into anyone in the game that seems to have a 100% perfect idea that fits. The closest thing that I've seen is if you have X number of chocobos near yours, you get 25% bonus for each one. But one thing I don't get is you can get 75% 5 times in a row, get nothing, then 25%, and again back to 75%. I've had all my guys keep there chocobos close together, and it still seems to vary. We saw 100% for 6 straight mobs, then last killed mob was 0%. I noticed while doing fates solo, and chocobo out, that often times I'd get 100% and indeed there was many chocobo's around not in my group, so there does seem to be some kind of a proximity thing going on with other chocobos.