For years, Democrats were generally aligned with labor, and broadly opposed to trade agreements -- remember that Hillary Clinton campaigned on rejecting the TPP [1], and it was unusual that Trump agreed with her, taking the issue away. Now, suddenly, the left is on the other side of the issue, because the current executive wants to restrict trade. It's nothing but realpolitik.
Also, not that long ago, it was the left that was advocating tariffs. For example, Obama in 2009 [2]. Admittedly nothing as sweeping or rushed as what is going on now, but still far from the party of free trade.
[1] https://www.politico.com/story/2016/10/hillary-clinton-trade...