Basement ac high pressure switch

[AFChap]

I replaced my original basement heat pump about 10 months ago with a new model 46515-811 ...an exact fit. The new unit has worked great, being used about 50% of the time since installation. Then yesterday I switched the unit on while doing some chores. First compressor started fine, but when 2nd tried to start it tripped the Surge Guard. Tried again, same thing. I had some trouble a month or so ago with my 50a circuit. This time it checked out fine but I changed to 30a, and then to generator just in case. On both 30a and generator the 2nd compressor simply failed to start.

Long story short, today I found the problem was the high pressure switch for #2 was tripped. Re-setting it restored normal operation.

Why would the high pressure switch trip? ...unit is clean, has good air flow, blowers are running as normal. And why would the switch being tripped trip the Surge Guard?

[MtnTrek]

Doubtful an armchair mech. might solve the total riddle.
The purpose of the High Pressure cut-out is to protect the compressor and components from just that, excessive head pressure. Excessive head pressure in itself causes increased amperage draw of the compressor. You noted most of the typical causes of excessive head pressure. Generally air flow is number 1, In a Heat Pump, depending on the cycle of operation (heating / Cooling) there could be any number of possible culprits.
With multi-stage capacity there must be some means of increasing air flow / volume when the second stage is energized. Via another fan(s) or increasing fan speed(s)
It's possible in moderate temps that a defrost cycle went awry, it's entirely possible that a reversing valve plunger hung during a portion of the cycle. HP's operate under a lot of variables, relays and controls, some can be... well, finicky. You'll no-doubt be monitoring it's operation closely could have been a fluke.
Best of luck.