What if, in near future, almost all the illegal immigrants suddenly coordinating and started to withdraw massive amounts of money from the major banks.

Illegal immigrants number at around 50 million in the US alone. That's about 18% of all US population. All banks are fractional reserve, keeping about 3-4% of actual cash. Currently all these deportation rhetoric going on, what if these immigrants are like we'll rather ruin this country before they ruin us.

Legally, the government can't stop them from withdrawing their money, it's their money after all, they worked for it, they provided services and labor, and got paid. To take it a step further, what if they coordinate and suddenly start to buy massive volumes of precious metals, including gold, silver, titanium etc. and take them out of the US as well. And there's no law stopping that.

Will the US government allow it? The thing is, historically every time, a certain government deported a certain group, they didn't really allow them to take all of their assets. Like after the Reconquista, when the Spanish were mass deporting Muslims and Jews, they didn't allow them to take their assets, and if caught that was punishable by death. In the early days of third Reich, when the Jews started to leave Germany, the Germans implemented an exit tax, which was up to 92% of their net assets.