A common justification I see for supporting Trump on libertarian websites is, "Trump will deport people who cost taxpayer dollars welfare money." I find that people often use "reducing the burden on welfare" to justify authoritarian beliefs.
The argument I heard more often, as someone who until recently spent a lot of time on libertarian websites, was that immigrants typically come from countries where socialism is popular. The majority of Libertarians reject that line of thought though.
This, at least in part, is a very “old school”, and not particularly libertarian, fear. See: Sacco & Vanzetti, et al.
