Yes, only putting one walnut in there is most definitely the reason. They will fight over the one treat, scream, try to rip it out of each other's mouths, chase one another, push the other(s) back, etc. You can get away with only giving one goo a treat so long as the other(s) are away and not paying attention, but if not, you have to give them all a separate treat or the shrieking goes on until everyone has finished the snack and over it. Next time, definitely put in two or crack one in half then put them in there to try and avoid the mini-brawls. Even so, if one finished before the other (or decided to bury it, or drop/leave it elsewhere), they might go and try to steal the other piece and the angry grunts and shrieking starts. They're such little monsters.
I've lived with twenty-two goos, nine of which are still alive with me, and it can get insanely hectic if I want to give them a snack, or else everyone starts screaming and are jealous that they don't have something yummy and so on. I have to make sure I have all if it in my hand and then quickly give everyone one to prevent the hollering (but usually there's a couple of large weep-weeps before everyone gets one or if someone walks past someone else).