Waterline on back of refridgerator keeps popping out and leaking

The water feed tube that goes through back wall of freezer to ice maker is most likely frozen. That is caused by leaky water valve. Thaw tube and replace valve

There should be a compression fitting that seals the water tube to the fridge. The fitting looks like a brass or plastic nut that the tubing sticks into. Shut off water supply to the fridge and carefully remove the fitting nut...there should be cone shaped compression ring inside the fitting nut. Slide the nut over the tube, then the compression ring over the tube. If the compression ring is tapered towards one end, then put the narrow end towards the end of the tube. Now stick the tube up to the fitting on the fridge where the nut screws on to the fridge. Hold the tubing firmly against the fitting and slide the compression ring and nut up to the fitting and screw the nut back on (compression ring should be inside the nut). You must hold the tubing firmly against the fitting until you get the nut tightened back down good with a wrench. This should take care of it.

If you find that the tip of your tube end is damaged, then you may have to cut off the tip of the tube to get it to seal and hold. If you find that the compression ring is damaged (won't slide over the tubing) or missing, then you can go to Lowe's or Home Depot and get a new nut and compression ring...they are cheap. You can also get new tubing there if you need it.