My dog suddenly started growling at other dogs at the park - what happened??

my 3 year old border collie mix Finn has always been great at the dog park. like legitimately great - plays well with everyone, never had a single incident in almost 2 years of going. then last month he started growling at certain dogs. not all of them, but maybe 1 out of 5. no snapping or lunging, just a clear warning growl.

the first time i chalked it up to a weird day. but its been consistent now and i'm kind of freaking out because it came out of nowhere. he's fine with dogs he knows at the park and still plays normally with some of the new ones. but then occasionally he just... objects to a dog.

nothing changed at home. same routine, same food, same everything. he had a full vet checkup 6 weeks ago and was totally healthy. is this some kind of personality shift? did i do something wrong? he's a rescue and i really dont want him to become the "problem dog at the park" situation