The left only became pro immigration to paint Trump as a racist just like they went from bing “anti-war” to sounding like neocons who want war with Russia because Trump wants peace.
A lot of the left is driven by, and funded by, the teachers’ unions (who are in turn funded by taxpayers); the pro-immigration stance came about to prevent shuttering of public schools due to lack of children. The whole funding mechanism for the Democratic Party itself would be in jeopardy.
Public school teachers are agents in every district, supported by the taxpayers they oppose on many issues.