Replying to Binding on right ski popping out

I have the Marker Griffon 12 on my K2 Extremes and this last Friday I went skiing and 3 times in the night my boot on my right ski just popped out of the binding. The first time I was just going up a slight incline and had to "walk" with my skis and it just popped out. The 2nd time I was just getting off the lift and I was getting ready to go down the run when it just popped out. The 3rd time I just got on the lift and about 10 feet after I got on it popped off. For the next hour or so of skiing was perfectly fine. Do you guys know what can cause this?

Thanks,
Bill

P.S. - This did not happen for about 2 hours into me skiing