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@Greyparrot
My personal theory is the support for the secure communities program
Obama expanded the secure communities program which helps to explain the uptick in deportations during his presidency. It was discontinued due to its unpopularity in indiscriminate deportations rather than only the criminals which explains the drop of deportations after the program was discontinued. The unpopularity of the program while it was active lead to the establishment of "sanctuary cities" which decreased deportations towards the later years of the program.
Trump revived the program upon becoming POTUS, however the sanctuary cities have not revised their stance on supporting the program, leading to decreased deportations.
At any-rate it's a better theory than waving your hand and asserting that there must've been more illegal crossings