Why would the cap on the power steering pump blow off and the fluid spray out?

I would be thinking that the pump is at fault, you can get the pump line pressure check just to be sure all is ok, but if it is a problem that is not constant this may not show much, but it is a test I would consider.

POWER STEERING PRESSURE TEST.†A power steering pressure test checks the operation of the power steering pump, the pressure relief valve, the control valve. The hoses. And the power piston. Basic procedures for performing a power steering pressure test are as follows: Using a steering system pressure tester, connect the pressure gauge and shutoff valve to the power steering pump outlet and hose. Torque the hose fitting properly. With the system full of fluid, start and idle the engine (with the shutoff valve open) while turning the steering wheel back and forth. This will bring the fluid up to temperature. Close the shutoff valve to check system pressure. Note and compare the pressure reading with manufacturer’s specifications.

Automatic trans oil type F is used, the pressure test will show a faulty relief valve in the pressure readings.