I'm having this problem right now with one of my bindings. The pins won't
stay seated in the slot anymore. My guess is that the slot wears and
won't hold the pin in place. I spoke to two ski technicians who said that
when it keeps popping out, the entire binding needs to be replaced.

I just came back from the ski shop. The guys are very familiar with
the problem. Instead of just selling me a replacement plug, they
further replaced the standard pin in the plug with a thicker one.