Power MOSFET blew up when increased duty cycle from 5% to 10%
If the damage isn't caused by overheating, I suspect that inductive voltage transients exceeding the MOSFETs voltage rating may be the issue here. In that case the culprit(s) may be the physical layout, component selection and/or the lack of a snubber network.

Where is the freewheeling diode D5 physically located? Across the motor terminals, right next to the MOSFET Q6 or somewhere else? Even if the inductive transients generated by the motor are clamped by D5 right at the source (motor), the parasitic inductance of the motor wires will still subject the MOSFET to a voltage spike of its own every time the current is interrupted. This is especially true if the motor wires are long and/or follow different routes (large loop area).  

Another potential issue could be the freewheeling diode's speed. The gate driver advertises a typical fall time of just 10 ns, while a fall time of 58 ns is advertised for the MOSFET (at 58 A, 125 V and a gate resistance of 3 ohm). This is slightly faster than the 75 ns reverse recovery time of that diode.



I'd add another freewheeling diode as close as possible to Q6, unless already present, in which case a snubber network should cure the problem. While the freewheeling diode would still take care of the majority of the stored energy, even a simple series RC snubber between the drain and source would take the fastest edge off that spike. 
Yet another modification could be to purposefully slow down the MOSFETs on-off transition by increasing R29, reducing the rate of change of current (and thus induced  voltage) and allow more time for the diode to switch on.