How to randomize the delay in Arduino Stepper code?
In setup(), add 

randomSeed(analogRead(0));


This reads the value of an analog input pin, which if not connected, will float to relatively random values between 0 and 1023. This "seeds" the random number generator so the pattern of random numbers you will get later doesn't always give the same results each time you start the sketch.

In your delay() call, just replace the fixed value with a call to the random(min, max) function, as:

delay(random(10000, 20000));