Gromit has it right. This resignation is nothing to do with disability, and everything to do with Europe. IDS is a famously fervent Eurosceptic.
It's massively disloyal to his leaders, positioning the PM and Chancellor as heartless, stupid or both, and himself as the opposite, all in an attempt to get more Brexit votes later in the year. As a double win, he is now free to argue for Brexit without the bother of having a ministerial position to distract him.