I’ve seen other rodents canibalize even those of their own species. This happens when there is overcrowding.
I saw it with voles we caught as kids.
We used to catch them and put them in a cage. If we didn’t feed them and they were overcrowded they would eat each other.
The article said the behavior occurred when there was an explosion in the population of voles.
The squirrels might instinctively see the voles as competition.
Just my unscientific opinion.