But the Democrats haven't made an¥ significant changes to immigration policy. In fact, they offered an bipartisan immigration bill that tilted significantly toward the Republicans' wishlist and the GOP rejected because Trump wants to run on immigration and giving Biden a political win in that area was unacceptable to thim (even though many on the left found the proposed legislation very disagreeable).
The main change at the border is a larger number of migrants. Migration trends are up globally, for a variety of reasons including climate change.