I see that a lot with students Marie. In their case it's that they are thinking hard about the steps of the frog kick and they put too much feeling into the first loading stroke, going backwards a foot, then they give a power stroke with poor ankle position and go forward a foot. Does make me laugh on the inside, deep inside where the poor peeps can't see it.
The clue is that you can do it without thinking, except when you are task loaded. That's a sign that you are begin to break down basic tasks into discrete steps, a common symptom of approaching task overload.
Relax and it should go away as your DS becomes more second nature to you.
Oh and just a point in general, the back kick uses the sides of the fin for the thrust, if you don't have rigid, wide-ish sided fins you will have to get creative.