No semicolon before [] is causing error in JavaScript

var a = [1, 2, 3, 4];
var b = [10, 20, 30, 40];
console.log([a, b].length)
[a, b].some(function(x) {
  x.push(x.shift())
});

I was extremely surprised today when this code caused

[a,b].some(function(x){ x.push(x.shift()) });
      ^
TypeError: Cannot call method 'some' of undefined

Obviously the JavaScript 'auto semicolon insertion' is not working as expected here. But why?

I know you might recommend to use ; everywhere to avoid something like that, but the question is not about whether it is better to use ; or not. I would love to know what exactly happens here?