Why is anti-siphon leaking with just one zone?

It is difficult to guess, but most probable cause: the anti-siphon is installed in the wrong place. Is it 6" above any other opening in the same zone? Check manuals, maybe your should be 12" (or 18").

Do you have a filter somewhere? If not, maybe some grain of rot or sand is entered on syphon and it make is more difficult to close.

Or there is a problem in your zone. Anti-siphon need pressure to close, maybe there is some leak in the zone, or you attached too many springers, so water has difficult to get on right pressure, and so it take time to close the anti-siphon valve.

Try to exchange the anti-siphon, from the leaking zone to an other zone (and contrary) and test. This could give you hints if it is something in the line or the anti-siphon.