Why does my hose connection leak when I turn on my outdoor anti syphon hose connection? The hose washer is new.

Are you 100% sure it's the actual hose/bib connection and not the antisiphon cap that's leaking?

It is the anti syphon that is leaking

OK because you said, "my hose connection leak " and that would be a different matter. An antisiphon valve has seals and springs that allow air into the valve when you shut off the water. Like any mechanism with springs and valves they can leak over time. These are usually cleanable if you unscrew the cap on the top, be careful removing the springs and s4eal, clean and reinstall. I usually try once then buy new if it still leaks.

I have tried to remove the antisyphon but with no success. Are they pinned?
Am I missing something?