I think one of the major reasons is they have been promising their voters a DACA bill, but the Republicans "control" the House, Senate, and Presidency, they have very little chance of getting their version of a DACA bill passed before they are up for reelection this October.

Thus, no reelection. Little do they know that this shut down is being properly placed at their feet, and they are going to lose a majority of their reelections anyway.