It's quite simple actually. To start off, we make polyurethanes from two monomers, a diol and a diisocyanate. Remember, electrons have negative charges, and the nuclei of atoms have positive charges. And we all know that negative charges and positive charges attract. These hydrogens are vulnerable, because they are bonded to oxygen atoms.
Oxygen is electronegative.
This is to say it pulls electrons away from other atoms. So it pulls electrons away from its neighbor the hydrogen atom. This leaves the positive charge of its nucleus unbalanced. The electrons would have balanced the positive charge with their own negative charges, they've been sucked away by the oxygen. This leaves a slight positive charge on the hydrogen.