subpanel breaker tripped as well as main breaker - should I be concerned?
Grats on your successful generator test :)
Whenever you have a bolted fault, you flow hundreds of amps.  The only thing that impedes flow is the resistance of the wires themselves, which you can look up on the internet.  &quot;Hundreds of amps&quot; is well within the range of the &quot;instant trip&quot; functionality of every breaker in the panel except perhaps the main.
Due to manufacturing tolerances and environment changes, &quot;instant trip&quot; has an acceptable range.  The acceptable range runs between 4x and 10x, where it becomes a guaranteed trip.
So for instance, a 500A bolted fault is a guaranteed trip for a 15A and 50A breaker, and a possible trip for a 100A main breaker.  Probably will not trip a 200A main.
If your short had flowed only 90 amps, both breakers would operate in thermal-trip mode (most likely). The 15A breaker would see a 6x overload, and the 50A breaker would see a 1.8x overload.  The 15A breaker would trip in a few seconds at most, and the 50A breaker would trip in a few minutes, based on the usual trip curves for breakers. In that case, the &quot;selective coordination&quot; you were hoping for would work as you'd expect.
However, all bets are off in a bolted-fault/instant-trip mode.