What could have caused my high-side MOSFET gate driver to break?
No decoupling caps, for one. You should always put decoupling caps in parallel with EVERY power-GND pin pair for EVERY IC unless you know specifically why you don't need to. There should also be a bulk decoupling capacitance.
Your problem is probably that high-side gate drivers runing off bootstrap capacitors can't work at continuous duty. They rely on the output node periodically going LO to allow a current path through the bootstrap diode then bootstrap capacitor to GND into order to refresh it. In a half-bridge this occurs whenever the low-side transistor turns on which happens in normal operation while commutating a motor or inverter.
In your case, you have it flowing through a 10K resistor which does a whole bunch of bad things. While it does provide a current path to recharge the bootstrap while the transistor is off, the current is severely limited, and while on, the boostrap current is drained.
WHen you put in a 1/2 Ohm dummy load, the charge current was probably too high and destroyed the internal bootstrap diode causing it to fail short.