It's because the far left and the far right are both made of up of people deeply disaffected by the status quo, and when those people talk they often find that at the very least many of their grievances overlap.

In terms of today's landscape there is a list of things like LGBTQ issues, race, gender equality, abortion, religion, etc., and if you avoid things on that list you'll find a huge overlap between the views of the far left and the far right. Both are broadly opposed to what's popularly called neoliberalism, the post-Reagan/Clinton post-cold-war order, and the reasons for this opposition overlap quite a bit if you again avoid the topics that I listed. From that perspective, blowing up the system is the goal. When they see trade policies like these crash the present system, they view that as a success because they think the current system is such a mistake that it must be smashed.

(I am not making a judgment in this post, just explaining the landscape.)