The power brakes are electronic, so the car stalling won't cause them to lose power. Even if there is an electrical failure, there is a boost reservoir that will assist for several stops. And if there is an electrical failure, it is likely that it would beep or store a code in the brake ECU. A dealer should be able to check for that.
If the boost reservoir is used up the brakes become soft for some reason. It doesn't make sense because in most cars the pedal becomes very firm if the boost reserve is used up. Try it out by pumping the brakes for a while with the ignition off.
I can think of one way that what you described could happen. Maybe there was a failure in the master cylinder somehow that lets the pedal go to the floor or be softer than normal. When you try to stop a second time, pushing the pedal faster could activate Brake Assist that automatically applies full braking power and activates ABS (it's annoying, mostly).
It doesn't make sense for the wheels to lock up. ABS should activate. If they did lock up there would be rubber skid marks on the road, and you would lose some steering control (not that you'd notice if stopping in a straight line). You would probably hear screeching tires, but I've herd it sometimes on smooth concrete roads in cars with working ABS and EBD (Electronic Brakeforce Distribution - uses ABS to balance braking between the front and rear depending on load and conditions). If they didn't lock up the truck would still come to a stop very quickly with ABS.
On one car I've driven, ABS didn't work under about 4mph, but I don't think that's the problem here.
I think in rare cases the automatic transmission can stall the engine. A sudden stop might do it. If it starts up fine after, I wouldn't worry about it unless it happens on a regular basis.