I had the front driver side axle disengage or "pop out"

Recently had the front differential assembly replaced, and now I have had the front driver's side axle disengage or "pop out." The mechanic who did the front differential is my nephew and is an ACE Certified mechanic and he said he is a bit baffled by this, but here is what happened: About 3 days after the work, I made a right turn and when coming out of the turn it just revved. I coasted to the curb and parked it. I spoke to my nephew who agreed to come look at it that evening, so I had it towed back to my house about 5 blocks away. While it was being backed off the flatbed in front of my house, the tow driver put it into 4Low which engaged (I hadn't thought of that) so I drove it into the garage. My nephew came out that night and within about 10 minutes got the axle to line up and re-engage, although he could not tell me why it would pop out other than maybe he didn't get it right when he did the front differential. Several days later it happened again, also when taking a right turn. I was able to get into 4Low and was limping home. On my way, I coasted in neutral down a long hill and into a left hand turn lane. While waiting for my arrow, I put it in Neutral and then to Drive (4H and I felt it engage. I let off the gas and immediately began moving forward. When my arrow came up, I proceeded home without further problems. I told my nephew this and he was shocked it popped back in on it's own. He will be doing Lower Control Arms and taking another look at it for me this weekend but I thought would ask to see what you think. Any feedback or suggestions are appreciated.