From the 1930ies there was a feeling it would be better to bring Jewish children away from the European mainland into safety against the upcoming anti-Semitism. Those children came mostly in a non-Jewish environment and some were even submerged in an other religion. Many years later they, their children or grandchildren wanted to find their Jewish connection again and would love to worship the Only One true God again. for that reason many look for becoming to be recognised a as Jew again, though these days they see the danger again of the uprising anti-Semitism, which makes them afraid to come into the open with their faith.
Looking at the results of two surveys (one from 2013 and one from 2017) which mainly looked at one group of "Messianic Jews" instead of looking at all sorts of millennial Jews.