If the ejector rod was partially unscrewed (and this happens all the time; I check them each time I clean a S&W), then it may have battered against the front of the ejector rod housing under recoil. That could cause the tip of the ejector rod to "mushroom" as your gunsmith described.
I have had a few S&W N-frames "lock up" because the ejector rods have partially unscrewed. It is just something to check as regular maintenance.
