H Bridge failing under load
Thanks for posting the question. I would like to know what do you mean by motor stalls under load? (During normal operation, the motor shouldn't stall)  What is the stall current of the motor? I believe the stall current is causing the Transistors to blow. What is the time duration for which the motor stalls? I think for the duration stall the Transistor is being operated outside its SOA region and causing the TIPs to blow. I am attaching an image of TIP 147s SOA graph.

The value of Ic corresponding to Vce = 20V is 6A from the above graph. During stall condition, I think the motor is drawing more than 6A of current for the duration of the stall causing the TIP's to blow.
Also here is a snip from the datasheet indicating the same.

Note that the stall current is always higher than the rated current.