Motor moves a little bit when I load program

I have two motors that move in a certain direction with an H-bridge, but when I load the program into the Arduino, one motor moves for like half a second, like a small shake, why does that happen? any idea?

#define E1 10  // Enable Pin for motor 1
#define E2 11  // Enable Pin for motor 2
 
#define I1 8  // Control pin 1 for motor 1
#define I2 9  // Control pin 2 for motor 1
#define I3 12  // Control pin 1 for motor 2
#define I4 13  // Control pin 2 for motor 2

char opcion;
 
void setup() 

{
 
    pinMode(E1, OUTPUT);
    pinMode(E2, OUTPUT);
 
    pinMode(I1, OUTPUT);
    pinMode(I2, OUTPUT);
    pinMode(I3, OUTPUT);
    pinMode(I4, OUTPUT);
    Serial.begin(9600);
}

void loop()

{
  opcion = Serial.read();
  switch(opcion)

  {
    case '1': 

    analogWrite(E1, 153); // Run in half speed
    analogWrite(E2, 153); // Run in full speed
 
    digitalWrite(I1, HIGH);
    digitalWrite(I2, LOW);
    digitalWrite(I3, LOW);
    digitalWrite(I4, HIGH);
    delay(9000);
    digitalWrite(I1, LOW);
    digitalWrite(I2, LOW);
    digitalWrite(I3, LOW);
    digitalWrite(I4, LOW);
    break;

    case '2':

    analogWrite(E1, 153);  // Run in full speed
    analogWrite(E2, 153);  // Run in half speed
 
    digitalWrite(I1, LOW);
    digitalWrite(I2, HIGH);
    digitalWrite(I3, HIGH);
    digitalWrite(I4, LOW);
    delay(9000);
    digitalWrite(I1, LOW);
    digitalWrite(I2, LOW);
    digitalWrite(I3, LOW);
    digitalWrite(I4, LOW);
    break;
  }
}

danteWinner:
why does that happen? any idea?

No.
At least not without code and a schematic diagram.
Read the "how to post" sticky first.
Leo..

In setup(), try digitalWriting all the control pins (except PWM) HIGH before pinModing them to OUTPUTs.
Or LOW if that don't work.

    digitalWrite(I1,HIGH);
    digitalWrite(I2,HIGH);
    digitalWrite(I3,HIGH);
    digitalWrite(I4,HIGH);
    pinMode(I1, OUTPUT);
    pinMode(I2, OUTPUT);
    pinMode(I3, OUTPUT);
    pinMode(I4, OUTPUT);

EDIT: And try another pin besides 13 for I4.

When power is applied all pins go to be inputs, therefore a lot of your motor control outputs float at this point. Also the bootloader toggles pin 13 as part of it's start up. Then your code runs.
So avoid pin 13, and put pull down resistors on you motor enable pins.

I'd wager it's pin 13 that's causing the problem, because it's used to blink the light for the bootloader. Use a different pin.