Dog used to spend most of his time playing with other dogs at the dog park, now he is mostly just barking at people?

So I have a 2 year old terrier mix. I’ll start by saying he has always had an anxiousness around new people. It takes him some time to warm up and he eventually does. We go to the dog park several times a week and he has a blast because he loves playing with other dogs and generally focuses on that and sniffing around, playing in the water, whatever. Every now and then he might find a person he would bark at - maybe someone who was particularly tall, or wearing a hat; but I could always kind of shoo him away and get him to go back to playing. But now all of a sudden he’s forgoing playing with dogs in favor of barking at people. All of them. Im not really sure how to address it? Obviously he has some anxiety so that’s part of it. But he’s also totally willing to take treats from people, and then goes right back to barking at them. I’ve told people not to give treats because I’m worried he learned that barking equals treats. He also will not let anyone pet him. Not sure where to begin with this. Any tips are helpful.