I remember doing the same thing in my earlier days of windsurfing. The down side of putting the back foot in first is that it is more difficult to get on plane (sinks the tail) and if you get in too soon it can cause the board to quickly head upwind if you put too much weight on the back foot.
I am guessing that you must be gaining some speed and are on plane when you get into the back strap. If this is the case, you are more likely to get out of balance and get tossed over the front while picking up speed out of the straps. This is where getting into the front strap first is a benefit. You become attached to the board earlier in the planing process and are less likely to get tossed in and can power up more quickly. Once in the front strap, it is a little harder to "find" the back strap, but it just takes a lot of repetition to make it easy.
When you improve your skills, you will want to get on plane very quickly or stay on plane after a jibe. Getting into the front strap fast will help with this process = doing a planing jibe.
Hope this helps -