Do the optocouplers get hot before they fail? After they fail, does the isolation barrier still hold?
It may be transients on the outputs of the optocouplers, not their supply rails. Try putting a TVS across the output of each optocoupler (from output to the emitter), as close to the opto as possible. Also put a schottky from the outputs to the positive supplies. Also I've seen many app notes advise a small resistor in series with the bootstrap diodes, so that reverse recovery currents are reduced and don't screw with the supply rails.
Finally, keep in mind that although your low side drivers nominally at the same voltage as the bridge ground, high transients can cause them to jump several volts, potentially destroying those drivers. Very large drives generally usefully isolated drivers on all the switches.