I think it's because young people feel no particular affinity with either political party and they want everyone to understand that.
And I don't blame them for the no affinity part. But as they get older, hopefully they will realize that they are Dems whether they like it or not, because in our political environment, sane=Dem and GOP = BSC at the party level at least.
This is also why I say to the Lincoln Project people (and CFord!) that they should just admit to themselves that they are now conservative Dems. There's no old GOP and it's not coming back. They are going to have to admit that they chose the wrong team way back when, and it's good that they are trying to make up for it but joining the other team seems more sensible than standing out in the cold throwing stones.