Critter behavior

What do Blue Jays have against squirrels?
I just saw three of them ganging up on one very fast squirrel. Why?  
Squirrels don't bother eggs do they? Jays aren't predators are they?
What do they have against it?

They both eat acorns.

M Ljin wrote:They both eat acorns.

It's spring and they were in maples. No oak trees within 500 yards.
?

The kind of Blue Jays we have are omnivores although primarily vegetable matter and insect eaters. Yes, they compete with squirrels for nuts, but squirrels here are known for stealing eggs and hatchlings, and may compete for good nest sites.

If food is scarce and they've got babies to feed, that may increase their aggression towards competitors.

Blue Jays are groupies and may hang in multigenerational groups, so mutual defense seems in character.

Pearl Sutton wrote: It's spring and they were in maples. No oak trees within 500 yards.     ?

Are maple keys fattening up in your ecosystem? I'm not sure I've seen either squirrels or jays eat them, but Steve Thorn has observed the squirrels feasting:
https://permies.com/t/110456/raining-maple-seeds-squirrels

Actually we warmed up then froze again exactly at the time to kill the buds the maple helicopters come out of. There will be zero helicopters this year in this area.
The trees fully leafed out way early due to all this.

what a timely thing. the day before yesterday I was putting up pea brush and noticed three birds (maybe ovenbirds? robin-sized brown birds, very normal birds around here) ganging up on another of the same kind. Actually I heard some weird peeping and it was the injured bird- it was pinned on its belly, its back feathers had mostly been pulled out and it was bleeding. They were on the roof of the house next door. When I stood up to get a better look they all stopped the attack and stared at me (at which point I thought "oh crap". they did not come for me, though, at least not yet....). Then they all flew away, including the bird being attacked! It was very, very strange.

Pearl Sutton wrote:Actually we warmed up then froze again exactly at the time to kill the buds the maple helicopters come out of. There will be zero helicopters this year in this area.  

I recall you have a lot of Maple trees around, so that may mean that you now have a bird/squirrel food shortage situation. It will be interesting how things all work out.

Generally, animals only fight if they feel they need to. It consumes energy and risks injury. You may never know what really started the squabble you observed, but I expect that one side or the other had clear reasons. We will never be sure what they were.