Once I was exiting a highway at 60 mph. I let the gravity (going uphill) and friction with the road make my car stop. Both brake and accelerator pedals have not been depressed for the duration of coming to a complete stop. Once at 0 mph, I pressed the brake pedal down. The engine stalled. I turned ignition key off and on, and the engine started as normal again. This is the only time the engine stalled.

What could be a reason for the stalled engine?

The only explanation I have is the gas, when refueling, may had some water moisture in it.  

Car: 1995 Lexus GS300 with 250k miles driven.

Prior to the issue: I hadn't driven the car for 2 weeks. Then changed oil, then refueled the gas tank.

Extra: Auto transmission. Brakes work fine. Engine is well maintained. Exhaust catalyst oxygen sensors give OBD1 trouble codes.