The border crossing disaster is the direct result of actions by DONALD TRUMP.
The most immediate and obvious cause is that, in a direct attempt to sabotage Biden’s ability to deal with border issues, Trump and his media abettors sent out the FALSE message that Biden supports “open borders” and that they would be welcomed. This is not true, but created FALSE HOPE and dangled a DISHONEST invitation, which Trump did on purpose to create more border chaos. With the humanitarian exception of UNACCOMPANIED CHILDREN and the legal exceptions for those who qualify as legitimate refugees, Biden is NOT allowing immigrants to enter illegally.
The second and most foundational cause is that Trump destroyed the solutions that Obama had successfully put in place. It takes years to build bridges, but minutes to blow them up. Obama built on solutions developed under Clinton and Dubya Bush, who had decent immigration policies, to address the systemic problems IN THE SOURCE COUNTRIES, whose indigenous populations, largely the ethnic indigenous Mayans of Guatemala, El Salvador and Honduras had been oppressed and persecuted by governments, including military dictatorships, that had been installed and propped up by Republican administrations under Reagan and Nixon, and earlier under Eisenhower under the leadership of the Dulles brothers (his secretary of state and CIA chief).
Under the economic assistance and stabilization policies of Clinton, Bush II and capped off by Obama to undo the damage of decades of U.S.-caused damage, illegal border crossings under Obama had been brought to a FORTY YEAR LOW.
Then, in a single day, Trump just cut off all aid. No phase out, just abruptly pulling out the rug. Just blew up the bridge that had taken decades to build and which Biden must now rebuild.