Why does the pop-up on my sink's drain keep popping up?
If you look under your sink, where the rod from the plunger enters the sink's drain, you'll see that it passes through a screw-on cap.  Tighten that cap a little (not too much!) because it has come slightly loose.  The cap holds a ball that the rod pivots around, so by tightening it you'll make the entire plunger assembly harder to move.  Play with it a little until you get it how you like it.