0 is the initial value of a.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce

It seems you messed up the sample tests, backup your code, click Reset, paste it and try again, they should look like this:

describe("Basic tests",function(){
  it("Mixed list",function(){
    Test.assertDeepEquals(gooseFilter(["Mallard", "Hook Bill", "African", "Crested", "Pilgrim", "Toulouse", "Blue Swedish"]),["Mallard", "Hook Bill", "Crested", "Blue Swedish"]);
  });
  it("No geese",function(){
    Test.assertDeepEquals(gooseFilter(["Mallard", "Barbary", "Hook Bill", "Blue Swedish", "Crested"]),["Mallard", "Barbary", "Hook Bill", "Blue Swedish", "Crested"]);
  });
  it("All geese",function(){
    Test.assertDeepEquals(gooseFilter(["African", "Roman Tufted", "Toulouse", "Pilgrim", "Steinbacher"]),[]);
  });
});

you are not required to (and should not) mutate the input matrix; your algo is wrong. try your function with e.g. [[1, 2], [3, 4]], it returns [ [ 1, 3, 1, 3 ], [ 2, 4, 2, 4 ] ], which is not the transpose of [[1, 2], [3, 4]]

What language?

Read this: https://docs.codewars.com/training/troubleshooting/#works-but-no

Don't use global vars, they keep their values between tests. If you call your function several times in a row in your IDE it'll fail there too.

Because of the array zero based indexing.