Question
Not sure if this makes any sense, but I have a series of 6 standard automatic antisiphon valves on a timer.  When I run the lawn cycle, the first valve doesn't shut off when it's supposed to, and it stays on for the duration of the next several valves thereby wasting water and lowering water pressure.  I quadruple checked the settings in the timer, and it is definately set to turn off after 10 minutes. Before I try rewiring everything I figured I would ask to see if there is a possible culpret.

Answer
It sounds like you need to adjust that valve.  Open the flow control on the valve all the way (this is the knob on top of the valve).  Then close it all the way, counting the turns.  Then open it back up halfway.  This is the right adjustment for the valves.