Getting in the zone either happens by chance, on any given day and there aren’t really any events that trigger it. It just happens. Sometimes you do everything right to prepare and have the worst day on court. Other days it feels like you’re not ready and then all of a sudden it clicks.
There are certain ways to trick your body, or rather your mind into helping you get into the zone. Mostly its about thinking about one thing and letting the body do other things by itself. I don’t wanna make this post long, so I’ll make a quick suggestion. Buy and read the book Inner Game of Tennis by Timothy Gallwey. Amazing book and will answer many of the questions you have.