Very weird bug / odd behavior of OUTPUT

Question

So, I have this code (Arduino Mega):

void setup()
{
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(6, OUTPUT);
analogWrite(10, 50);
  analogWrite(11, 50);
}

very basic motor speed control. Now when I do this (nothing else is connected to board, only 10 and 11 PWM control pins):

void setup()
{
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(6, OUTPUT);
analogWrite(10, 50);
  analogWrite(11, 50);
  digitalWrite(6, 0);
}

motors stop. As if those analogWrites are ignored. Is this some exteption about Arduino board? Or is it just a bug? Again, nothing else is connected. Only 10 and 11 PWM outputs. Pin 6 is free. I tried puting digitalWrite above analogs - nothing.