Why is my 5eat legacy gt stalling in reverse

Hey guys kind of new to the forum and a new Subaru owner well my first and I love it but I just have one question, my legacy gt stalls in reverse like the idle gets really low and the car starts shaking before it felt like it just wanted to turn off. But, in the past couple of days it actually did turn off, but after I turned it on again it ran fine no problems what so ever it only happens when I turn on the car for the first time of that day after I drive it more it works going in reverse again no problem so it's just like a one time thing. Please if you know what could be the issue causing this please let me know I just find it weird wen I does that.