Introducing players with (formerly) big reputations is the obvious starting point when searching for reasons, but xG also tells a clear story. As in Union had a ridiculous overperformance last year. 50 goals from 35 xG, 62 points from 42 xPTS [understat.com]. Sheraldo Becker alone doubling his xG value with 10 actual goals. In terms of xG they already had the worst attack last year and their xGA was pretty good at 42 xGA, but nothing spectacular either: Köln's was even better for example.
This year their xG moved up to second worst team in the league, but in reality they went from absurd overperformance to moderate underperformance.
If we look at their individual stats two players stick out:
Sheraldo Becker, the huge overperformer of last season, is without a goal in the BL so far. Now before we play blame the Dutch guy we should also consider that he's only got 0.98 xG to his name, which probably means Union's counter attacks aren't what they used to be - which I guess is an obvious fact, if you consider that they are trailing every game sooner or later.
And then there's David Fofana, the kid they loaned from Chelsea, who also sits on 0 goals, but actually has 3 xG to his name.
So their attack was always kind of shit, but they relied on overperformances and set pieces to grab goals anyway. But this seasons their output has been more in line with xG, when xPTS already put them 6th from bottom last season. Add some bad luck and a young striker, who doesn't score, to the mix and it's not the biggest surprise this situation deteriorated into a big losing streak.