Just because. She did it once, and now does it. Or she saw another dog do it. Or the silicon switch inside her head switched.
3 of our 6 dogs do it, the other 3 don't. our Shih Poo never lifted his leg until he saw the beagle I got lift his. The Shih Poo was neutered young and so were the 2 big dogs. They simply put their legs apart and go. The Shih Poo only saw them go that way so he simply emulated the pack behavior.
After he saw the beagle lift his leg, I saw the light bulb go on.
One of the big dogs (Rhodesian Ridgeback/Doberman cross) gets all four legs going when he is flinging dirt. The beagle uses his back 2. The newest guy, an 8 mos old Shih Tzu gets all four going when he urinates.
There is something from back in their evolutionary history that causes them to do it and all they need is a trigger to get going.
