Pigeon attacking another pigeon

We watched a pigeon attack a young pigeon (fluffy, rotund) by jumping on its back and pecking furiously at it on the back of the young bird's head and around its eyes and it only stopped when I opened the kitchen window and shouted.  The young bird prior to this has been lying flat and only  moving its head to look around in the shade of the hedge at the side of our house. Half an hour later the young pigeon had made it out of the  garden, across the street and it was in the neighbours' garden lying and looking around. It has now disappeared. 

My question is? Do you think it is likely the attack was territorially motivated: or was it a parent trying to scare some activity into a young bird or was it just a straightforward malevolent attack on a younger bird?