It's hard to imagine how your feet slip off the pedal backwards. About the only thing I can imagine is that you must be letting your heel fall as you push down on the pedals or you are doing more pulling on the upstroke than pushing with the other foot on the downstroke.
Maybe just your calf is out of shape letting your heel fall and needs to get stronger to compete with the other leg muscles powering your bike. Possibly a way too low saddle might contribute some toward this.
Is your cadence real high when this happens? 80 rpm and better as a routine cadence is when I realized that I really needed clipless. Though I was flat pedals at the time. Never used traps or clips.
And as
zandoval said, maybe you just need to get back in the grove.