To prevent the signal lines from going above Vcc by more than a diode drop. This protects the chip inputs (or outputs, in this case). It doesn't matter WHY the signal line goes above VCC (ESD, whatever), it is necessary to protect the chip.
Given that each output is also fused, we can assume it is intended to protect the device from miswiring. Inductive kickback is handled inside the driver chip for the case where the output goes below ground. These external diodes clamp when the output is somehow pulled above Vcc.