This isn't a product of any recent developments; it's taken a long time to get where we are, and it's taken a long time for us to be conditioned enough toward socialism to accept the raw emotion surrounding the issue, in light of the wealth of rational arguments (economic and historical) against it.
Reduced to the bare-bones, I'd say the two core issues are:
1. Removing entitlements
2. Enforcing borders
Permanent, life-long entitlements are just a bad idea, and this is just the latest evidence of it. Treating poverty like an incurable pathogen, and believing that it will be assuaged by a lifetime ration of "free" money is an idea that could only have been birthed in the minds of marxists. Only they could survey the dynamics of the world that surrounds them, ignore them, and attempt to mandate a perverse status quo on the populace.
Enforcing the borders is obvious. While I recognize that the world is full of despotic, corrupt, dirt-poor excuses for sovereign nations, that isn't our doing. The world isn't going to be a better place if we make our own country a despotic, corrupt and dirt poor excuse for a sovereign nation.
These can serve as a litmus test for any political hopeful. Don't support enforcing the borders and eliminating entitlements? I guess you aren't going to do anything. Nice knowing you!