They are starting to figure out they are the losing end of this food fight.
Or they figure they're giving Trump the rope they can hang him with.
Once they give the executive branch the money, they've washed their hands of the problem. After all they gave him more than he asked for - their hands are now clean.
It is the executive branch who is actually stuck with the problem. And it may well turn into a major problem no matter how well it is handled and the Dems can blame Trump for the deaths that are coming.
That's how I see their action.